Kirk's biography in The Making of Star Trek stated that he was 34 years old and had commanded the Enterprise for "more than four years." It was also the original source of the claim that he was the youngest starship captain ever
Yeah I might be mis-remembering the stuff about Decker - the Phase II solits only state that he was in his thirties.
Kirk states his age at 34 in season 2 I think. and it's generally accepted that the 5-year mission came to an end in 2270 so he would be 37 by the end of the 5 year mission and presumably 32 at the start. Thus with the re-fit he might have been as young as 30 when he took over the Enterprise. That's still pretty young.
I really thought Riker referred to the fact that he was still younger than Kirk was when talking about his career path with his Dad or Troi. I can't be bothered going back to check.