You ask the potential client their age. It is illegal to lie about your age online. You must be 18 years old to have a PayPal account, but it is also against PayPal's Terms to distribute NSFW products (including art). For any other type of chargeback, contact your bank for a dispute as you have proof of age, delivery, and consent. This can help you in legal means.
I agree with Celestina, too as it is great information.