When your child becomes a legal adult, you don't stop being his parent, but you do have to adjust and modify your parenting style. You wouldn't discipline a toddler the way you discipline a teenager, because each stage of development presents unique challenges that require different approaches. Adult children will not respond well if you implement the same punishments and expectations that were the norm in their adolescence. The challenge is to adapt your discipline approach to respect your child's new-found freedom, while clearly defining what behaviors and actions will not be tolerated.
Establish clear and concrete boundaries together. Boundaries are especially important if your adult child is still living at home, in order to prevent conflict before it arises. Draft a written contract with your child that covers issues like curfew, friends staying overnight, car use, household expenses and chores. By helping to create these guidelines, your child will feel empowered and respected, and there will be no grounds for argument if an agreed-upon rule is broken.
Let the past go. Don't use past bad behaviors or poor judgment as a point of contention in present disagreements. Your child will resent the implication that he has not matured and will become immediately defensive. Forgive past mistakes, and don't let them color the way you view your child's actions today.
Respect your child's independence. Realize that while you may disagree with your child's behavior in certain instances, it is no longer your place to intervene. If your child is not causing direct harm to himself or others, ask yourself if it's really productive to raise your objections. Choose your battles wisely, and don't alienate your adult child by disciplining him in situations where it's less right and wrong and more a matter of opinion.