Welcome Mr Fender,

Ah the dilemma of too many women!

I imagine that things with the 18 yo won't turn into anything serious because of the distances involved, right?
Also the 28 yo is no means guaranteed to end in a serious relationship because you are going away, correct?
Have you told any of them that this is going to end up in a serious relationship?
If you haven't told them or hinted at it and they know that you'll be either a long distance or moving then they probably don't expect anything serious.
Be honest with them both so if you do decide to go and see the 18 and 28 asks something about whether you have seen anyone while you've been seeing each other tell her don't lie.
I have a girlfriend now but at the start I really wasn't sure because I was out every weekend pulling girls. I went and slept with someone else and I felt guilty even though we weren't in a relationship we kept seeing each other and I decided to be with just her. She asked me if i'd seen anyone else and I told her and she's fine with it. The old me wouldn't have.

The best thing is to be honest but not knowing the situation entirely I can't say what you should do for sure.