What about a child who ages over 18 and wants to continue sharing prior purchases with the family, but use his own payment method for future?

Here is one way to do it: He can stay on the Family Sharing group. Going forward, he can use his own funds to purchase and redeem gift cards to his own Apple ID. so that he always maintains a credit balance sufficient to cover his own purchases. By staying in the Family Sharing group, he also will have continues access to the purchases of other members.

For further details, see information in here: How apps, content, and subscriptions from Apple are billed - Apple Support

• • "If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on."