It’s called “boundries”, & if they’re respected a good relationship can be had. If not, then, the parents may end up having to be cut off, because eventually an ‘adult child’, has to claim their adulthood. Believe me, I know… I only wish I had figured it out sooner.
As a parent of adult children, it’s hard sometimes to know when to say something and when not to say something. I err on the side of not saying something and then I’m blamed for not being more “involved.” So sometimes? You just can’t win. So I just try to be there when I’m needed, hope that someday I’ll be wanted, and stay out of the way as much as possible.