The “Share purchases on iPad with family members” chapter of my iOS 12 iPad user guide tells me Family Sharing is still limited to children, and still not suited for use with special needs adults.
The problem is that “Ask to Buy” is available only for 18 and under. Since all purchases go to the “Family Organizer” Ask to Buy is the only way for the Organizer to control what family members purchase intentionally or accidentally. Since it’s turned off at age 18 “Family Sharing” is effectively for parents (who presumably share expenses) and children (who can have Ask to Buy).
This means that Family Sharing is not helpful for special needs dependents (guardianship status).
This is unchanged from iOS 11. I’m not surprised, Apple doesn’t want Family Sharing to be widely adopted beyond the target group.