I totally agree that asking for proof of age is a legitimate thing to do, just like a bar tender or off license would. If they really insist that they are over 18 and you're still not convinced then they could scan a copy of their passport or something and email it to the team as proof. We may want to keep a copy of these somewhere just in case. However if in doubt, although it seems unfair to ban someone who really is over 18, it is always best to err on the side of caution.