I have had many friends ask me to write a parenting book on parenting adult children, since many of us have adult children living at home. It is difficult because as parents we want to parent our kids like we did when they were younger. The one thing I can tell you is there is no way you can parent your adult child like you did when they were teenagers. One very important thing to remember is that your child knows you. That statement means they know how you think, what you think. what your responses are and what your expectations are. You don't need to give them your opinion, unless they ask.
Many of our adult children are living at home I believe because life is more difficult than it was for us. Most are not just graduating from college in four years, but working towards master's degrees. Jobs are not plentiful. Housing is expensive. With the divorce rate so high, I think many are taking their time to commit to another person. What our adult children need from their parents is love, support and respect. They need to know you have confidence in them. You must remember that your child is an adult and when you give them that respect, they will in turn, respect you.