I don't know.
Emperor Frederick II (died 1250) was born 26 December 1194. His mother Constance was born 2 November 1154 and thus was 40 years, 1 month, and 24 days older than her son. It is said that, as was customary, she gave birth with many witnesses to make sure there was no doubt that Frederick was her child. But Frederick's enemies sometimes denied that he was actually not the child of his parents, which was also rather customary, and to make it seem more doubtful they sometimes exaggerated Constance's age, claiming she was 50 or even 60.
Eleanor of Castile mentioned by Lars Bosteen apparently lived to be 49 years old. Since she died 28 November 1290 she should have been born between 29 November 1240 and 28 November 1241. Her youngest child, King Edward II of England, was born 25 April 1284, and thus when Eleanor should have been between 42 years, 4 months, and 28 days and 43 years, 4 months, and 27 days old.
Eleanor of Castile was a great great granddaughter of Eleanor of Aquitaine also mentioned by Lars Bostten. Eleanor of Aquitaine was allegedly 13 in the spring of 1137, thus being born between about March 21, 1123 and June 21, 1124; and also allegedly turned 14 in 1136, thus being born between January 1, 1122 and December 31, 1122. And she was also allegedly 82 when she died 1 April 1204, thus making her born between 2 April 1121 and 1 April 1122.
Thus Eleanor of Aquitaine was probably born between 1 January 1122 and 1 April 1122. Her youngest child, King John was born 27 December 1166, and thus when Eleanor should have been between 44 years, 8 months, and 26 days and 44 years, 11 months, and 26 days old. But if Eleanor was born as late as 21 June 1124 she would be as young as 42 years, 6 months, and 6 days when John was born.
Eleanor of Aquitane was a great great granddaughter of Duke William V of Aquitaine. Emperor Henry IV was a grandson of Duke William V of Aquitaine. Emperor Henry IV was the father of Agnes of Waiblingen who was supposedly born sometime between the summer of 1072 and early 1073. We might assume that Agnes was born between about 21 June 1072 and 21 March 1073.
Agnes's youngest child by her second marriage was Gertrude, wife of Duke Vladislaus II of Bohemia. Gertrude was said to be born about 1118, thus making Agnes about 44 years, 9 months, and 11 days to 46 years, 6 months, and 11 days old when Gertrude was born (if in 1118). But Gertrude is also said to have died age 30 on 8 April 1150, thus being born 9 April 1119 to 8 April 1120, making Agnes about 46 years and 19 months to 47 years, 9 months, and 18 days old when Gertrude was born.
Agnes was the great great grandmother of Emperor Frederick II by her first marriage.
Maria Dobroniega of Kiev (died 1087) was the wife of Duke Casimir I of Poland. Her father St. Vladimir I, Great Prince of Kiev (c. 958-1015), had many mistresses as a pagan ruler before converting to Christianity. Vladimir's second (but first Christian) wife was Anna (born 13 March 963), daughter of Emperor Romanus II. There is a slight possibility that Maria was the daughter of Anna and Maria's descendants would thus be descended from the Macedonian dynasty of Emperors.
If so, Anna would probably have been quite old when Maria was born, since Maria's youngest child was born in 1046 to 1048. Anna married Vladimir in 988, so Maria (if Anna's daughter) could theoretically have been born as early as 989, thus living to be about 98 and being about 57 to 59 when her youngest child was born. Vladimir died 15 July 1015, so Maria could have been born as late as April 1016, making her only about 30 to 32 when her youngest child was born. If Anna was alive in April 1016, she would be about 53 years and one month old, which would make her a very old mother if she had a child then.
One source says that Anna died in 1008/11. If she died giving birth to Maria, Anna would have become a mother aged about 45 to 48, and Maria would have become a mother aged about 35 to 40. Another source says Anna died in 1022. Any evidence that Anna was the mother of Maria would be evidence that one or both of them was a mother aged over 40.
Duke Albert II of Austria (1298-1358) married Johanna of Pfirt in 1324, They had several children who died young. Their first surviving child, Duke Rudolf IV was born 1 November 1339, their youngest Child, Duke Leopold III, was born 1 November 1351 and Johanna died 15 November 1351.
Johanna's father, Count Ulrich III of Pfirt, died in 1324, so theoretically Johanna could have been zero years old when married, 15 when Rudolf IV was born, and 27 when her youngest child Leopold II was born. If Johanna was at least 12 when married she would have been at least 27 when Rudolf IV was born and at least 39 when Leopold III was born.
Johanna's birth year is given as 1300, which would make her 50 or 51 when her youngest child was born, though some suspect she was born later.
Johanna was a great great great great great great granddaughter of Agnes of Waiblingen.
These examples are enough to show that some women did give birth in their forties during the middle ages, and also that it is hard to be certain who was the oldest.