Eye contact generally means that someone (anyone, guy or girl) is interested you in some way. It might be for a nice conversation, or they think you look nice, or they want hot steamy sex. The age of the people involved doesn't really matter. But there are certain looks, maybe an up-and-down body look, that if repeated might be more sexual. Humans aren't too clear on sexual signals, not like most other animals.
Guys do look at girls and sometimes think of sex with them. If the girl is under 18 I think it is "creepy" (can't think of a better way to say it other than putting it in quotes) for a guy more than 2-3 older to be in a relationship with her. But as you get into the 20s and 30s, age isn't so important. (I'm 37, my wife is 26, and we got together because of overwhelming mutual interests.)
And again, a look is a look, and it probably is more specific to the person than how old they are.