Chekov lies about his age?! I never thought he was that vain!Was Chekov's age ever established on screen in TOS?
He claimed to be 22 during "Who Mourns for Adonais".
Beat me to it by 2 minutes
Seriously, this is probably just one of those things that can't be explained by the time travel. There are a few others too.
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Well... CAN'T is a pretty strong word. I just thought of a way. But I'm not going to go down that road 'cause it would just prove I'm a scary obsessed Trekkie loser who spends way too much time thinking about this. I'm trying to maintain deniability on that score.