For me, I would say that is ok in that our vendor supplied a build day manual, and within it, there were waiver forms that were intended for volunteers under the age of 18...it just required the parent had to sign them in and sign the waiver.
I'd say 15 is OK in that a). they're as tall as adults and so equipment isn't going to be accidentally swung around at hip height and catch a child in the face (e.g. like a 9 year old height would be). And they are old enough to understand that this isn't a play time, it's time to work.
We had a couple of teens helping, particularly when it came time to move the bark around, and they were a big help.