The title pretty much says it all. What is the oldest treaty (of any type) still being enforced between two political entities (i.e., nation-states, cities, etc.)?
Both Parties Sovereign and Remain Sovereign
Anglo-Portuguese Alliance (or Aliança Luso-Britânica), ratified at the Treaty of Windsor in 1386, between England (succeeded by the United Kingdom) and Portugal is the oldest alliance in the world that is still in force – with the earliest treaty dating back to the Anglo-Portuguese Treaty of 1373.
English aid to Portugal went back much further to the 1147 Siege of Lisbon.
Sovereign Parties create a new Sovereign entity
The Federal Charter or Letter of Alliance documents the Eternal Alliance or League of the Three Forest Cantons, the union of three cantons in what is now central Switzerland. It is dated in early August 1291.
One can argue that this document is not in force today, being superseded by many other documents defining Switzerland.
Between Sovereign and his Subjects
Magna Carta Libertatum (Medieval Latin for "the Great Charter of the Liberties"), commonly called Magna Carta, is a charter agreed to by King John of England at Runnymede, near Windsor, on 15 June 1215.
It is hard to argue that it is still in force, because
the original 1215 charter had concerned the medieval relationship between the monarch and the barons, rather than the rights of ordinary people, but the charter remained a powerful, iconic document, even after almost all of its content was repealed from the statute books in the 19th and 20th centuries.
Neither Party Sovereign
Paréage of Andorra was a feudal charter signed in Lleida on 8 September 1278. It codified a lay and ecclesiastical agreement between the Count of Foix, Roger-Bernard III, and the Bishop of Urgell, Pere d'Urtx, establishing their joint sovereignty over the territory of Andorra.
The decisions of ...
The First Council of Nicaea was a council of Christian bishops convened in the Bithynian city of Nicaea (now Iznik, Bursa province, Turkey) by the Roman Emperor Constantine I in AD 325.
... are still binding on Christendom.
The Pact of Umar, is an apocryphal treaty between the Muslims and the Christians of either Syria, Mesopotamia or Jerusalem that later gained a canonical status in Islamic jurisprudence... attributed to 9th century Mujtahids ... the pact contains a list of rights and restrictions on non-Muslims (dhimmis).
I highly recommend a careful reading of the long list of the "rights and restrictions" (e.g., non-Muslims are not permitted any weapons and are required to wear a distinctive dress - cf.).
Between People and God
Covenant of Abraham, Isaac and Jacob with God (~3,500 years ago). Still fully in force - see Judaism.SE.