According to most wiki pages surrounding the character,
Mordenkainen was born in 509 CY. The last entry that used the Greyhawk calendar year was when mentioning how he started to shave his head in 591 CY.
With this information in mind, if most adventures in the Forgotten Realms take place during 1491 DR, then how old would this NPC technically be during the Curse of Strahd adventure?
My guess is around 212 years old.
According to the Vistani at Tser Pool (on p. 36-37 of the adventure), he apparently came to Barovia over a year ago. The current year in Barovia is 735 (according to the "Barovian Calendar" sidebar on p. 26 of the adventure), which may just translate to 1491 DR since adventurers start in Daggerford. If we are to use the conversion table provided in "5e official adventure timeline", 587 CY is 1361 DR. CY consists of 364 days in a year while DR consists of 365, so if present campaigns take place in 1491, then that would mean 130 years have passed by (making 717 CY the current year in the world of Greyhawk).
If this is correct, how is it possible for a human to even live this long?
My sources for information: