You know, it’s not like he can’t do PE right now if he wants to, using the info freely available here. He just can’t be a member until he’s 18, and I think that’s OK. No reason for Thunder to risk problems by having minors joining up.
As to the rejoinder, I did a quick look around and didn’t find an obvious disclaimer about having to be 18. If it’s there, I missed it and he might have too. If there is one and he blew it off and joined anyway, then he deserved to get his wrist slapped.