18 is just an arbitrary number indicating legal maturity, not sexual or mental. He's not talking about a hairless kid. One day the kids jailbait and it's, "creepy," to think about him but a mere 3 months later it's OK to think he's hot? Some people just mature a lot faster than others and you can't tell who is really legal and who isn't without a birth certificate. There are 18 year olds that look 14 and 14 year olds that look 18. Hell, in New York the age of consent is 17 and in others it's 16. He's legal in those states. What's creepy is a mature adult lusting after an immature kid of any age.
When I was an adolescent I lusted after kids my age or older, does that make me perverted? Now I'm 2-3 times their age and they look way too young to be attractive to me. Maybe, however, Luke is in his late teens and this guy looks to be about his age and he finds him attractive. I don't see anything perverted or creepy about that at all.