According to Family Sharing from what i can see, ASK to BUY is automatically defaulted for any child under 13, and it can be turned on for any child under 18. It implies that once it it turned on, it can remain on for any member still on the plan regardless of age.
So, if you would be able to work with Apple Support to temporarily change the 21, 18, and 18 year olds' birthdays back to age 17, turn on ASK to BUY, and then fix their birthdates back to their actual ones, seems like that would meet your needs?
You could contact Apple Support to see what they could do for you in this unique situation:
Apple Support Numbers WorldWide
One other alternative would be to take them off of Family Sharing, but provide them with a limited Fund each month to be able to purchases what they wish. An "Allowance" so to speak. Certain types of Apps, Music, Websites, etc. can be limited by setting on Restrictions on each device (which only you know the passcode to)
Since I'm not sure what else you are using Family Sharing for, this may not be a viable alternative.
Howerver, I think your situation is unique enough that Apple would hopefully find a way for you to your the ASK TO BUY feature with any one that is on your Family Sharing account, if you deem the need for it.
Best of luck!
Report back with your outcome, if so inclined!
Cheers,
GB