As usual, lol, I have to disagree with you on a lot of these kinds of issues.
I'm a heterosexual male and my observation is that women in their late teens often aren't fully developed, just as most men aren't fully developed in their late teens. If I consider athletes, it seems that most people, men and women, are at their prime in their mid to late 20's. But, that's totally different that attractiveness.
Attractiveness is a different matter all together. IMO, the average person hasn't lost any attractiveness by the time they in their mid 30's, for many not much change until their mid 40's.
I also agree with Epsy, the things that I do to maintain myself, I do for myself, and no one else. And, btw, my body today is better than it was 10 years ago, and I've continued to improve my physique throughout my life.
I don't know what the future holds, but if my father's genetics hold try for me, I'll be mistaken for someone in my 60's, when I'm 80 and I'll have a 34" waist at that age too. Of course, he never worked out to stay fit nor ate healthy either. So, perhaps, I might even be better...