-have sex with someone your casually dating?
It depends on when you and the other person feel it is right.
-consider someone your dating as your boyfriend/girlfriend?
When you have talked to them and you have decided together to be officially dating.
-start sleeping over at each others places?
When you're comfortable doing it. My boyfriend slept over before we started dating - - - because we were friends before that.
-meet their parents?
I think maybe once you are official, or maybe it depends on if they LIVE with their parents. My boyfriend (of a year and 4 months) lives in Illinois, and I live in Michigan, and we met at school in Illinois (although not where he is from). Due to that fact, I didn't meet his parents until our 1 year anniversary. It may be weird to some people, but not for us.