Take the Hinges Off the Door

*Disclaimer-small rant below*

I don't understand age differences in relationships. Today when i was waiting for some friends to come play disc golf with me two cars pulled up. In one car was a man that seemed to be in his early thirties and the other a young girl who seemed to be high school age, maybe a college freshman if you are really pushing it! So the young lady gets out of her car and starts to walk toward the car of the creepy old army man. When she gets there they kiss a few time and there is a lot of physical contact which makes it very clear that they have some sort of romantic relationship. This girl was attractive and seemed to be relatively normal. The guy was in no way attractive and in my observations and interactions with him lead me to believe he was kind of a jerk.
First off i don't really understand how this happens. How can you have a relationship with someone that wasn't or maybe just barely was born in the same decade as you. Maybe its just the age i am but i cant really see dating a girl that was within one to two years of my age either way. I just don't understand how a relationship with a difference more than that even works.
The second thing that really grinds my gears about relationships like that is that for the guys from 19 years old until they turn at least 21 it makes it pretty hard to even get girls to notice you. It all just makes me frustrated partially because i just don't understand it and partially because what i do kind of understand i just think is lame.
That is all
Peace