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 accounts for people who enter into the contract under 18 yet turn 18 during the contract period.