In my experience, it started to feel like everyone living in student digs during the 17-24 phase. We all did our own thing. They all knew how to use the cooker and washing machine, and it was a case of every man for himself. It was a little wild for a while, but everyone wanted to be independent, ultimately. It is tough to get out, but everyone has managed it, in general by moving out with a partner. It is extremely difficult financially these days, so they have all come and gone a bit. We never charge rent as it's usually because they have fallen on hard times for one reason or another.
It does take really good communication skills to ride over that change between child and adult, but we aim to base it on mutual respect, which has worked well for us so far.
It's a whole new dynamic to navigate, and you have to kind of disconnect the parent/child relationship in order to reconnect as adults. I don't think that comes from threats, laying down the law, etc, but by keeping in mind that it's difficult for everybody.
There are basic respects like keeping each other informed re comings and goings, regular partners being welcome to stay over, but nobody unexpected. They tend to lurk in their rooms, and that is their own affair re levels of grot ( ime they grow out of the teenage level of untidiness by mid 20s) Closed doors are respected. DH and I generally cook and they join us or not, but they keep us in the loop.
In short we have found it's a continuation of the good relationships we've built since childhood, and the whole thing is quite stable now. Currently adult child free, and that's better all round, but I really want to emphasise that early 20s are a time for ongoing negotiations regarding how to live in a respectful and loving manner.
All the best OP!