I don't think that age is a factor here. Any pistol that you own and shoot would have to be on a FAC and there's no lower age limit on that.
Owning a BP, however, (like any other gun) does have age restrictions. But they can be used in the company of a responsible adult (that lets out most of us on this BBS ) who has the weapon on his FAC.
That's how I see it, however, if I'm wrong then somebody might just mention it.