Unis have this age dimension going on all the time. Remember that uni is not all about undergraduates, as well as a few mature undergrads there are students in their late 20s and 30s doing masters courses and PhDs, some of them have interaction with younger undergrads in seminars and extra catch up classes and so on.
There's nothing fundamentally wrong with getting to know younger students, it's not like you are dealing with people who are underage, however unis do tend to contain examples of 'that guy', and you don't want to be 'that guy'.
'That guy' is usually a guy who is a bit of a loser with relationships, he has struggled to find anybody to go out with him and feels like he's getting older so he thinks now he's in a university environment, there will be more single females around because they are younger, so he will start hanging around them trying to befriend them. Usually this is the type of guy who is a superficially 'nice guy' type but a bit manipulative and will get bitter and angry if a girl turns down his advances. He will probably think that 18 year old girls will be a bit naive and impressed by his extra years of age and life experience, however the girls will have a better social radar than he suspects and they will soon suss him out and chat amongst themselves about him being a bit creepy.
Ironically the older guys who are most likely to catch the undergraduate girls attention, are the ones who don't actively try to befriend and chat them up, ie they just have contact with them through tutorials or whatever, and if they are reasonably good looking and confident etc then the girls chat will be speculating on is he single, talking about how they like going to his classes because he's a bit of eye candy etc etc.
I used to take a first year tutorial group and I enjoyed the teaching part and the banter with undergrads, obviously you notice when there's fit girls in the group but I reckon it's not the proper thing to try and take contact outside of the classroom. One time I was out in town and one of the girls from my group was there, she came over and got me to come and meet her friends, we chatted for a bit, she was a bit drunk and giddy, then she started dancing flirtatiously with her hands on me and then tried to get off with me! I broke it off and just said "sorry, but I'll get in trouble if I do anything like this" and both of us got pretty embarrassed and made a bit of awkward small talk before I went back to my friends. I then was not looking forwards to going in for the next tutorial but (unsurprisingly) she didn't turn up, there were only two more sessions of the semester and she didn't turn up for either of them. Even though it wasn't my fault I felt bad that I'd put her off from coming back in to the tutorials and I also hated the idea that it would have probably been talked about and become 'gossip' in the group.