I think it's fine to hang out with friends late into the night.
What are your reasons for saying it's fine?
I just think that it is fine.  Parents should accept things once someone turns 18.
It's okay to live your own life, but do so responsibly.  Now that you are 18, you are responsible for the consequences of your actions, so keep out of trouble, and don't take unnecessary risks.
It's not their business to tell me what to do.
If you live with them, they can still tell you what to do and it is their business. They will allow you more freedom, but don't betray that trust by getting into more trouble.