10-12 in human years. I believe the only picture of him in that flashback was the Briarstead overlook and he seems fairly tall in that (for a 10-12 year old, anyway). But again, inconsistent/inaccurate heights and appearances are something I will gradually rectify as time goes on and I get better at drawing. I don't mind if players have different interpretations of how old the elven characters are (unless Sunny whips out her tits, then she is 18+), but if you want a more accurate estimate, it's better to evaluate their behavior before their appearances.
But on the note of elven vs human years, there doesn't need to be any confusion IMO. Sure, elves would definitely age differently than humans - a 40 year old elf will not look like a 40 year old human, which is nothing new in the world of fantasy. But there is no concept of "elven years" as I see it, so when the protagonist was 10, he was just 10 and wouldn't look entirely out of place among human 10 year old children.
Also, any notions of when elves truly mature would depend on the elven culture rather than an exact science; some elven snobs might say that elves haven't matured until they've reached the age that a human can't and so on, but that doesn't mean that 100 in human years = 18 in elven years. It's the equivalent of someone IRL saying that you're not a man until you have hairs on your balls.
If you want, ping me on discord and we can talk about it more
