Or to give equal time, people can ignore the line about being just 17 in ST09.
No, they can't. Walter Koenig was 30 when he did "Who Mourns for Adonais," so it's just as easy -- maybe even easier -- to accept him as 26 in that episode as the stated 22. But there's no possible way to accept Yelchin's Chekov as only 13. It's hardly a symmetrical situation.
If I can accept a 55 year old James Cromwell playing a mid 30's Zephram Cochrane then I think I can manage accepting a 13 year old year old Chekov plated by a slightly older actor. It certainly makes the "I can do that" running through the ship scene much easier to accept. Just think of him as Wesley Crusher who actually went to the Academy instead of becoming an acting ensign.
But he would seem to be a 13 year old Academy graduate. I seriously doubt he'd be on starship duty even if he was able to accomplish it.