I started at 13, no problems... Sometimes though it can depend on a person's pain threshold- having said that, even if it does hurt at first, they will get used to it the more they play.
Some sites like the one local to me provide "body armour" for the younger kids to make sure they don't hurt to much it may be worthwhile taking a look around some fields that may provide this service.
It should be fine, explain to him that if he is hit it will hurt and if he's fine with that then your all good, if not be cautious you don't want to be in the middle of a battle field having to console you little cousin ruining everyone's game