How to know he’s lying about his age?

I’m really in love with my boyfriend and we’ve been dating for almost a year but I think he’s lying about his age. When we met he said he was turning 21 but recently I asked what year he was boring and he was like “umm I don't know 1999, 1998?” How doesn’t he know? He’s “21” and his birthday is November 6 so wouldn’t he be born in 2000? He did drugs before so maybe that’s why he looks older , but recently I compared his face to another 21 year old and he looks older. Is there a way to find out his age without asking? I honestly don’t want to make a big deal about it but I’d like to know before I make my decision to leave or not. I love him but we are toxic.
How to know he’s lying about his age?
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