Casual: going to eat and occasionally having him pay... Sex after a date... Not meeting family... Not talking about relationships.
Serious: having him pay when you go out. Kissing in public. Holding hands in public. Meeting family. Calling frequently. He brings you up in future plans. Sex is romantic and sensual...not two bunnies getting it on.
I think that a lot of men do not want serious relationships when they are young. They want to experience so much in the single life. However when you fall in love with someone, you can't help it. No matter what age you are. :)