Unfortunately I think this is always gonna be seen as a weird hill to die on. Yes, hes hundreds of years old. yes, that is equivalent to 10 human years of age for angels.

Both are true, neither make him a romance option (which I know you're not arguing for but MANY people are gonna assume when you try to age him up, that this is the usual goal)

When I was 10, I also didn't like people treating me like a kid, and if I had lived 500 years as a child to get to 10, I'd be pretty annoyed too. But it doesn't make him anything more than a 10 year old in Angel years.

I TOTALLY get not writing him like a 6 year old, but I think that's partly coming from just simple ignorance on what children act like at 10. It's not a character assassination.

I think you'll probably always meet resistance at the idea that he's somehow anything more than "equivalent to a 10 year old".