Aren't paypal basically saying that they are holding the money in the account for a 180 day period, incase their is any reported fraud against Mitchells account, then they will release the money ? (copy paypal quote below)

your balance will be held for 180 days from the date of the last payment that was received into your account. At that time, any remaining funds will be available for withdrawal.

So, because he is not 18, and you have to be 18 to hold/operate/open a paypal account, they have chosen to hold the money for 6 months and then release it to you.  

Mikes advice is spot on, but if u have to be 18 to hold a paypal account (which is in its paypal user conditions) they may legally be within their rights to hold the money...which sux, but if their terms n conditions cover this, u may have to wait 6 months, OR they just might refund the money to the pinball machine purchaser. ? I dont see this happening, they will hold it for the 180 days then release it only to you i feel.