I was 13 when I first started questioning my sexuality but ended up settling on the one that seemed to fit the best (considering at the time I thought you were homo, hetero or bi, only three options). I didn't find out about asexuality until I was 20 or so and now identify properly. Funnily enough to your post, I'm agender.

I came out to my mother as gay when I was around 16 or 17, which went badly. I did come out to my brother as asexual later on, which I don't think he understood but he did accept, but after my first disasterous coming out to my mother I didn't bother doing it twice.