In nature, youth is associated with the ability to breed successfully, and a greater chance of survival/strength/longevity. As soon as animals are capable of breeding, they do.

Just because we’ve built a civilisation, it doesn’t mean those baser instincts are eliminated. Human males are designed to be attracted to the physical changes present in post-pubescent females, so they end up being attracted to that + youth.

There are many more reasons like taboo etc, but that’s the core evolutionary / biology reason.