If anything, Chekov's age was a retcon. The writers wanted Chekov on the Enterprise, and they changed what they had to in order to make it work. If you need to excuse it somehow, say Chekov was bullshitting when he stated his age during TOS - he hardly said it under oath.
Meh, it doesn't really matter I suppose. My issue is more that the gang's ages seem to have been squeezed together more so they have less diverse histories and experience but this is a movie franchise - how much screen time are the supporting cast going to get anyway?
I would have preferred Chekov to be absent completely personally - he wasn't around in TOS season one, he didn't add anything crucially important to the movie, most of his role, apart from talking in a silly Russian accent, could have been passed to a different character. Batman works just as well (and some say better) without Robin.
While I understand the deisre to shoehorn in the big 7, the result is that female characters got squeezed out. I'd have brought in Rand instead and then introduced Chekov in a later movie. I realise that that people say they didn't know that there was going to be another movie but even if that had proved the case, Chekov's absence would still have been understandable. Rand's absence is about as important as Chekov's would have been in the grand scheme of things but the wider issue for me is the absence of women in a franshise that is meant to espouse equality. I think Chekov's absence would have made room to redress the imbalance slightly.