I may be being very simplistic, but the signs of puberty must be present for anyone to be thinking they're 'hot', surely?
Ok reading that I probably am being simplistic but I think that's ok for a basic rule of thumb isn't it?
Developing from a child's body into an 'adult'/ mature body is what puberty changes are all about, and although during/after those changes teenagers are still not fully adult in many ways, at least they've getting some of the hallmarks of sexual maturity which is surely what starts to make them 'fanciable' on a physical level?
eg growing breast tissue if you're a girl, needing to use deodorant, voice breaking for boys etc etc.
Although this is complicated by the trend towards earlier onset of puberty, and younger and younger children having body changes when they are still just children. However, I would have thought that that was the issue for most, that the child has signs of sexual maturity and growing up, when they're not that age and definitely shouldn't be treated as if they are. Rather than the situation you describe when the young person hasn't had those changes yet?
Anyway, it's the middle of the night and I'm rambling! But basically, the thing that makes me dubious about the situation is what exactly is the 18 yr old finding sexually attractive if the young boy in question hasn't hit puberty yet in many ways?
It would be different if they were close in age but at different stages on the teenage puberty spectrum.
Perhaps it's less questionable if the younger teen has other traits that are the focus vs body, like a brilliant mind?