FINANCES - Once your child has become an adult, you are really not financially responsible anymore unless they are disabled or still in school. Help them to find a job or begin working towards a career so that they can establish independence. It is not helpful to allow your adult child to be idle so ensure that they are either working or attending an educational program. If they are working and you all agree that they can live at home for a period of time, it is appropriate to charge room and board. That will help your child to understand that there are responsibilities in life. NEVER co-sign for anything because you could very well end up with a debt that you didn't want or need. Also, do not give any money to the child. This only encourages him/her to expect more than they can earn and sets them up for disappointment in the future. (And NEVER borrow money from your adult child).
OTHER RELATIONSHIPS - Your child has the right to make mistakes and often those come in the form of unhealthy relationships. You likely do not want them to tell you about the poor choices that they think you have made so do not be critical of theirs. Being judgmental might actually end up with the opposite results to what you desire! Also remember, the person who your child is dating might end up becoming the parent of your grandchildren. If you are asked for an opinion, however, it is acceptable to give it. Just be careful with the wording that you use and how you express it!
HOUSE RULES - It is best to communicate your desires clearly. For example, you may not want your adult child to use drugs or sleep with someone who s/he is not married to in your house. If that is the case, you have a right and a responsibility to tell your offspring before it happens. This may create some tension or distance between you for a while so make sure that you understand the consequences and can not only can set the rules but also be consistent about them so you don't "cave" just to keep peace. Some rules from childhood don't really count anymore though. Your adult child does not need you to set curfews. If you cannot live with worry or sleep lightly, it might be best for the "all-nighter" personality to find a separate residence.
HOLIDAYS - If your son or daughter is in a serious relationship or marriage, you will need to share them with the other family. There may be times that they will want to have a Christmas or other times with their partner and children rather than with you - a situation which you will need to respect. It is not fair, however, for them to never schedule holidays, vacation times or celebrations with you during the year or to be with one family more than with the other. It makes sense that you want to be with them for the good times but this is not "all about you" and you all need to be fair.
Your adult child might not act like an adult at times and this can be confusing and hurtful. It is not up to you to change them and, in fact, you might as well realize that you can't change other people anyway. The hope, however, lies in the fact that all green things grow. Give your child some time and you will likely be pleased with how much they have learned and matured.