Family sharing ask to buy for adults?
Is there a way to turn on "ask to buy" for an adult in my family?
No, it is only for those under 18, though I guess it could be for older too as long as you hadn't turned it off.
Approve what kids buy with Ask to Buy - Apple Support
Who can use Ask to Buy
Familes can use Ask to Buy after they set up Family Sharing. The family organizer can turn on Ask to Buy for any family member who isn't an adult. It's on by default for any children under 13. You'll be asked to set up Ask to Buy when you invite anyone under 18 to your family group.1
If a family member turns 18 and the family organizer turns off Ask to Buy, the family organizer can't turn it on again.