Arm hair?

Hey all, I was just wondering if anyone knows the typical age that a child grows arm hair.  Does that come with puberty, or younger?  I feel like I had arm hair when I was like 8, and I didn't go through puberty until I was a whopping 17.  My 4 year old niece has arm hair, but I don't know if it just varies from child to child?

The reason I am asking is because my son has alopecia and is completely bald.  I am just wondering if he is going to have hair anywhere else on his body.

Thanks!