The minimum age is 18, i.e., old enough to sign a contract and be legally responsible for what they do online. Younger individuals are permitted to use eBay if they are supervised by an adult, but I suspect in that case the adult has to be the primary account holder.
As one example of this individual's lack of readiness to be an eBay seller, he apparently hasn't realized that eBay sellers cannot accept payments through PayPal. Since he's your buyer, it's not an issue since buyers can still use PayPal to pay for their eBay purchases.
