Ask to buy
Is there a way to have ask to buy ask to buy for people over 18 in the group
No, it can only be turned 'on' for people under 18 - from the page that you posted from :
Ask to Buy is on by default for any children under 13, and you'll be asked if you want to enable Ask to Buy when inviting anyone under 18 to your family group.* If you turn off Ask to Buy for a family member after they turn 18,* you can't turn it on again.