girls enter puberty so much earlier than boys, their capable of reproduction anywhere from 10-12. When you talk about "prime reproductive years," know that it includes girls as young as 10. So... stop using that term. I am giving you the benefit of the doubt and inferring that you actually mean girls that are on the very cusp of womanhood, like, ages 17-19, but others may not.
Women are hot, girls are not. Some girls can appear to be older than they are, and it's confusing when confronted with a genuinely attractive woman that is not yet 18, because we're caught between the confusing notions of "I am attracted to this person" and "this person is not yet old enough to to be engaged with in a socially conscious manner." It's not wrong to find the person attractive, it IS wrong to engage with them in a manner reserved for those that are fully realized adults. For my purposes, I'm putting adulthood at around age 22-25, when your brain is pretty much fully developed.
So with that being said, No, Jerry Seinfeld didn't do a bad thing by thinking a 17 year old girl attractive, he DID do a bad thing by engaging with her as if she was a fully formed adult.