The oldest medieval European monarch lived to be about 91 years as far as we know, though there is a slight possibility of older European medieval monarchs up to centenarians
[Gediminas (c. 1275-1341) Great Prince or Grand Duke (or maybe Great King) of Lithuania, only lived to be about 66, but many of his descendants lived much longer.]
46) King William the Lion of Scotland (c. 1143-1214) lived to be about 71.
45) Rene (1409-1480) Duke of Anjou & Lorraine, Count of Provence, and briefly King of Naples, lived to be 71 years, 6 months, and 22 days old.
44) Charlemagne (742-814) Emperor and King of the Franks, lived to be 71 years, 7 months, and 26 days old.
43) Abd-ar-Rahman III (889/91-961) caliph of Cordoba, lived to be 70 to 72.
42) Empress Zoe (c. 978-June 1050) was Empress from 1028 to her death, sharing power and the imperial title with her three husbands and her sister at various times, and living to be about 72.
41) Wladyslaw II The Elbow High (1261-1333) King of Poland, lived to be 71 or 72.
40) Vladimir II Monomakh (1053-1125) Grand Prince of Kiev, lied to be about 72.
39) Boleslaus III (c. 965-1037). Duke of Bohemia twice, lived to be about 72.
38) Emperor Andronikos II Palaiologos (25 March 1259-13 February 1332) lived to be 72 years, 9 months, and 19 days old.
37) Brian Boru (c. 941-1014) King of Munster 978-1014 and High King of Ireland 1002-1014, lived to be about 73 when killed.
36) Mael Sechnaill mac Domnail (949-1022) King of Mide 976-1022 and High King of Ireland 979-1002 and 1014-1022, lived to be about 73.
35) Albert (c. 1338-1412) King of Sweden 1364-1389 and Duke Albert III of Mecklenburg-Schwerin 1384-1412, lived to be about 73 or 74.
34) Emperor Michael I Rhangabe (c. 770-844) lived to be about 74.
33) Lewis II the German (c. 202-876), the first King of Germany, lived to be about 74.
32) Robert II Stewart of Scotland (1316-1390) lived to be 74 years, 1 month, and 17 days old.
31) Edgar II Aetheling (c.1051-c.1126) was the rightful heir of King Edward the Confessor in January 1066, but ambitious and powerful nobleman Harold II Godwinson was elected king by the nobles. After Harold was killed at the Battle of Hastings Edgar was elected King in October 1066 but submitted to William the Conqueror in December 1066. Edgar was alive in 1125 aged about 75, and there is a small possibility that he might possibly have been alive in 1158 and 1166, aged over 100.
30) Emperor Basil I (811-886) lived to be 74 or 75.
29) Mieszko III the Old (c.1126/27-1202) High Duke of Poland, lived to be about 75.
28) Ottokar I (c. 1155-1230) Duke and King of Bohemia, lived to be about 75.
27) Emperor Manuel II Palaiologos (1350-1245) lived to be 75 years and 24 days old.
26) King John I of Portugal (1358-1433) lived to be 75 years, 4 months, and 3 days old.
25) Queen Joanna (6 November 1479-12 April 1555) of Castile, etc. known as Juana la Loca, was Queen Regnant of Castile, etc. for 50 years, though her husband, her father, and her son ruled for her. She lived to be 75 years, 5 months, and 6 days old, though some would put that in the Renaissance era.
24) Empress Theodora (980-1056) not only was Co Empress from 1042 to 1050, but also sole Empress and actual ruler from January 1055 to 31 August 1056, dying age 75 or 76.
23) King Alfonso I (1106/09/11-1185) of Portugal lived to be between 73 and 79 years old.
24) Emperor Justin I (450-527) lived to be 77 years, 5 months, and 30 days old.
23) Emperor Frederick III (21 September 1415-19 August 1493) lived to be 77 years, 10 months, and 28 days old.
22) Eric of Pomerania (1381/82-24 September 1459) was King Eric III of Norway (1389-1442), King Eric (XIII) of Sweden (1396-1439), King Eric VII of Denmark (1396-1439), and Duke Eric I of Pomerania (1449-1459). He lived to be about 77 to 78 years old.
21) Emperor Romanus I Lecapenos (c.870-948) lived to be about 78 years old.
20) Durad brankovic (1377-1456) Despot of Serbia, lived to be 78 or 79 years old.
19) Emperor Nikephoros III Botaneiates (c.1002-1801) lived to be about 79.
18) Berenger I (c. 845-924) rival Emperor and rival King of Italy, lived to be about 79 years old when he was murdered.
17) Wladyslaw II Jagiello (Jogaila) (1352/62-1434) Grand Duke of Lithuania and King of Poland lived to be about 72 to 82 years old. Son of Algirdas.
16) Sancho VII (1154-1234) King of Navarre, lived to be 79 years, 11 months, and 21 days old.
15) Boris I Michael (died 902) Khan and Prince of Bulgaria (852-889), lived to be about 80 years old.
14) King Malcolm II (c. 954-1034) of Scotland lived to be about 80 years old.
13) Vytautas The Great (c. 1350-1430) Grand Duke of Lithuania, son of Kestutis, lived to be about 80.
12) Mieszko I tanglefoot (c. 1130-1211) High Duke of Poland, lived to be about 81.
11) Algirdas (C. 1296-1377) Grand Duke of Lithuania, lived to be about 81.
10) Sigismund the Old (1467-1548) King of Poland and Grand Duke of Lithuania, lived to be 81 years, 3 months, and 0 days old, though perhaps in Renaissance times.
9) Gruffudd ap Cynan (c. 1055-1137) King of Gwynedd, lived to be about 82.
8) Svitrigaila (in or before 1370-1452) Grand Duke of Lithuania, son of Algirdas, lived to be about 82 or more.
7) King John II (1398-1479) of Aragon, etc. lived for 80 years, 6 months, and 22 days.
6) Emperor Flavius Petrus Sabbatius Justinianus Augustus (c. 482-565) lived for about 83 years.
5) Edward Balliol (c. 1283-1367), briefly King of Scotland 3 times, lived to be about 84 years old.
4) Kęstutis (c. 1297-1382) Grand Duke of Lithuania (brother of Algirdas) lived to be about 85.
3) Stefan Nemanja (c.1113-1190) Grand Prince of Serbia, lived to be about 86.
2) Emperor Flavius Anastasius Augustus (c.431 to 518), lived for about 87 years including some in the middle ages.
1) Emperor John VI Kantakouzenos (c. 1292-1383) lived for about 91 years. Louis XIV is famous for being succeeded by his great grandson. In 1376, 22 years after John VI abdicated in 1354, his grandson Andronikos IV (1348-1385) and probably also great grandson John VII (1370-1408) were crowned - they didn't invite ex emperor John VI to the ceremony but threw him in prison instead. Some historians believe John VI was a few years younger.
[Added Dec. 21 2019. I read somewhere that Edward Gibbon doubted the birth year of John VI. John VI (c. 1292-1383) ruled from 1347-1354 and abdicated, becoming a monk under the name Joseph. Stefan Dusan (1308-1355) was king of Serbia from 1331-1355 and claimed the title of Emperor of the Serbs and the Romans from 1346-1355. His son Stefan Uros V (1336-1371) was emperor after him, but the empire fell apart into various semi independent lordships. Dusan's brother Simeon Uros )c. 1326-1370) proclaimed himself Emperor of Serbs and Romans in 1356, but ruled only Thessaly. His son John Uros succeeded him but abdicated as Emperor about 1373, and became a monk with the name Joseph, dying in 1422.
So there was an emperor John VI who reigned from 1346-1355 and died as a monk named Joseph in 1383, and an emperor John Uros who reigned about 1370-1373 and died as monk named Joseph in 1422. And if someone confused the two of them they might think that an emperor had lived to be 130 years old which was a suspiciously long lifespan.]
Queen Brunhilda (c.543-613), born a Visigothic princess, was never a monarch. But she did serve as regent for her son King Childebert II (570-595) of Austrasia from 575 to 583, for her grandson King Theudebert II (586-612) from 595 to 599, and for her great grandson King Sigebert (601-613) in 613. Even though she was killed when only about 70 years old, I can imagine that some people believed she must have been a lot older.
Enrico Dandolo (c. 1107-1205) Doge of Venice, lived to be about 98 and led the 4th Crusade aged about 97.
Albert Azzo I, Margrave of Milan, ancestor of the Welfs and of the House of Este, died 20 August 1097. He was allegedly born either 10 July 1009 (dying age 88 years 1 month, and 10 days) or in 997 (dying age 99 or 100). Emperor Frederick I was his great great grandson.
Liudolf (c.(805/820-866) Duke of Saxony, married about 830 to Oda (died 913), daughter of Billung. Oda may have become a child or teenage mother, but their older son Bruno (c. 830/840-880) indicates Oda could have been born about 800-825). Oda was allegedly 107 when she died in 913, thus allegedly born 805/806. Her grandson Henry I became King of Germany in 919 and her great grandson Otto the Great became emperor in 962.
There were many monarchs whose lifespans are little known.
For example, there are many websites and books that claim that Llywarch Hen ("The Old"), an ancient British noble and/or monarch of a small kingdom died in 500 aged 150. But Wikipedia, using more reliable sources, guesses he was born around 534 and died around 608 thus aged about 74.
I don't have time now but I will later add many other examples of celtic monarchs who might have lived very long.
More celtic monarchs who may have lived long.
Morgan Hen ("The Old") who united the Welsh kingdoms of Glywysing and Gwent into the kingdom of Morgannwg or Glamorgan, supposedly reigned from 942-974. One manuscript of the Annales Cambriae says in the year 973/974/975 "and Morgan died", supposedly Morgan Hen. A much later and less reliable source claims Morgan died in 1001 aged 129! Morgan was the maternal grandson of Rhodri Mawr (c. 820-878) and thus should have been born about 860-900, and been aged about 70 to 115 if he died when the Annales Cambriae says.
Morgan Hen's great great grandson Iestyn ap Gwrgant lived from circa 1045 to 1093 according to Wikpedia. Descended from Morgan Hen in four generation gaps, he should have been born about 960 to 1060 and aged about 30 to 130 during the Norman invasion about 1090. Much later legend claimed that after being dethroned he died as a monk aged 121!
King Arthur is supposed to be been killed at the Battle of Camlann, which the
Annales Cambriae date to 537/538/539, and Geoffrey of Monmouth to 542, however reliable those dates may be.
Some late medieval stories claim that Arthur was almost a century old when he died. That seems unlikely, but Anastasius (c. 431-518) lived to be 87, official and general Petrus Felix Liberius (c. 465-c.554) lived to be about 89, Justinian (c. 482-565) lived to be 83, and the official and general Narses (478/479/480 to 566/574) lived to be 86 to 96.
Arthur's contemporary Maelgwn Gwynedd who died in 547/548/549 according to the Annales Cambriae may have lived to be quite old. The statement that the reign of Maelgwyn was 146 years after his great grandfather Cunedda was old enough to lead a migration to Gwynedd suggests that the generations between Cunedda and Maelgwn were very long and/or that Maelgwn lived to be very old, while the dates of Maelgwn's descendants down to his great great great great grandson Cadwallader suggests that the generations between Maelegwn and Cadwallader were very short and/or that Maelgwn lived to be very old.
02-02-2018. Legendary British ruler Vortigern might have lived to be very old. The Anglo-Saxon Chronicle entry for 455, written centureis later, says:
A.D. 455 . This year Hengest and Horsa fought with Wurtgern the
king on the spot that is called Aylesford. His brother Horsa
being there slain, Hengest afterwards took to the kingdom with
his son Esc.
The Historia Brittonum, also written centuries later, dates the beginning of Vortigern's reign to 425, while the Pillar of eliseg, also centuries later, claims that Vortigern married the daughter of Magnus Maximus (c.335-388) usurping emperor from 383 to 388. If all those statements are accurate Vortigern might possibly have been a septuagenarian or older.