I can't think of a legal reason for this policy. Normally, one must be 18 years old to enter into a contract, so the policy of preventing users under 18 from using the service is rational. However, if a user under 18 turns 18 years old during the contract, the contract is "ratified" and becomes valid and enforceable. It's as if they were 18 years old all along. So, there is no legal reason for Paypal to terminate acc…
Depending on where you are, I think this is not entirely true. IANAL, but I think that in many/most US states, contracts with minors are not presumptively void, they are voidable at the option of the minor (or their guardian, I imagine). Of course, this is a pretty big downside for their counterparties, so it makes contracts with minors pretty unappealing, obviously.