Parenting adult children is a new ballgame. Just because your son or daughter turns 18 or 21 doesn't mean they stop being your child. It doesn't necessary mean they'll even leave the nest.
In my part of the country, the average age an adult child moves out of his parents' house is 28. Yes, you're reading this right. Twenty-eight.
As the mother of five with three of them older than 21, I can testitfy to this. My 26-year-old son and 24-year-old daughter still reside at home.
On the one hand, I enjoy having them close. They are, after all, my children. But on the other hand, they present new parenting challenges. Yes, they still have to be parented!
The primary problem is that they believe that, since they have reached the magical age of 18 or 21, they can do as they please, when they please, as much as they please -- and damn the house rules. Some feel that they don't have to help with house and yard work, or at least not much. Some don't think they should pay rent and expenses. Almost all believe they can come and go as they please and stay up all hours of the night. Of course, some are better than others, but I haven't met one yet, mine or my friends' adult children, who truly act grown up while they still live at home with mommy and daddy. They're too busy enjoying the best of both worlds.
Tough love is probably called for. Perhaps the landlord trump card has to be played. It's a hard balancing act.
Mutual respect is necessary for adults to co-exist. The sooner they learn this, the better for everybody in the house.
Don't expect too much. If they have younger siblings still at home, they are likely to interact with them as they always have, whether good or bad. Even though they believe they're all grown up, they haven't learned how to cope in the world on their own, pay their own (full) rent, household expenses, and other living expenses because we -- their parents -- are there for them to fall back on.
This isn't to suggest that you should kick them out or expect them to grow up overnight, but you should be aware that your job isn't done. Even when they move out and support themself, your job isn't over. It will never be over. However, it usually eases.
Although there are challenges and stresses when adult children refuse to fly from the nest, enjoy this extra gift of time. Some children leave the day after graduation never to return. I was one of those. So was my oldest son. Merely know that you have new challenges to face, and new parenting issues, when your adult children choose to live with you at home.