Re: STAR TREK INTO DARKNESS - Grading & Discussion [SPOILERS]
The age thing is such an old chestnut, and there are nits to pick at both ends of the specturm. Kirk was feeling old at 53 in TWOK, and he was basically put out to pasture at 62 in TUC. However, Picard was already 59 when he took command of the Enterprise in 2364. It's hard to rationalize Kirk being old compared to that.
I think Starfleet forced Kirk out (just my speculation).
"If I hadn't tried, the cost would have been my soul." - Admiral James T. Kirk, Star Trek III: The Search for Spock