Can you set up “Ask to Buy” if your children are over 18?

I would believe so as per.....

If you turn off Ask to Buy for a family member after they turn 18,* you can't turn it on again.

From -- Request and make purchases with Ask to Buy - Apple Support.

No, you can only initiate "Ask to buy" with people under 18.

Once a child turns 18, they will still be required to Ask to Buy until you remove that. Once you do, you can't add it back.