This Old Turk

[Ludens]

I recall someone posting that he had a king well over 100 years old. This was because he was leading a crusade that had stalled somewhere along the way, and the leaders of crusades don't die of old age.

By the way, if you make sure the heir is married, it is well possible that a new set of (not-yet adult) heirs will appear once he gains the crown, thus lessening the risk of having old kings.