Can you explain to me why his age matters
Well, for starters, it's illegal for him to have a PayPal account. Should anyone contact Paypal about JJ, his account is dead. Funds frozen. He hasn't said who his "business partner" is and whether or not they're handling the finances.
It was illegal for him to create a Reddit account when he did.
As it has been mentioned elsewhere, it's illegal for him to enter into any business contract due to his age... and should he do a runner, good luck going after a minor.
In this hobby, 100% of people who have tried to pull scams on me were underaged. So yes, I do have actual experience to feel like there's something to be wary about here. Since he IS underaged, the burden of proof that his GB should be legitimate is completely upon JJ should anyone raise concerns.