Kirk's 25 for the bulk of the narrative, which makes Chekhov, at 17, eight years younger than him. I have no idea how that maps to the original.

At least Kirk's life isn't the only one that's gone differently. The destruction of the Kelvin would have affected more people than just him, so I approve of changes that imply other people have followed different paths, too. (Maybe Chekhov's parents got together several years earlier than they would have?)