Girls- Do you act differently around guys when you already have a boyfriend/husband? When you're older?

I'm asking this question because, going back to college recently, I noticed 2 ladies in my English classes who act completely differently towards me than the rest of them. One in particular never makes eye contact with me, and basically acts like I don't exist even when we are put in a 3 person group to talk about a book. It's like she doesn't want anything to do with me (she doesn't know me though, we're pretty much strangers.) At first I figured they were like snobby, but then I developed a theory that type women are already involved with a guy in a relationship or in marriage and so they no longer have any interest in guys.

I guess I'm trying to figure out this 'type' of girl who appears extremely confident, indifferent towards guys in general yet still somewhat friendly with other girls, doesn't make eye contact with guys, or when they do interact with guys it's like they're acting like the guy is their brother, you know, acting sisterly.

The second part to the question I guess, is do girls act noticeably different in their late 20's early 30's compared to late teens early 20's?

Is this type woman married? Older and confident? Bitchy? Snobby? Lesbian? what? hehe