Without going into the fertility bullshit;
Female bodies do indeed tend to look better/be naturally in good shape at a younger age, and much sooner so than men.
Basically, any female after having fully gone through puberty will be attractive to most if not all (heterosexual) males, and they are without very much effort. You have those that are naturally beautiful with tight bodies, and they can indeed thrive on that until their early twenties, which is when the non-working out bodies will start to fall apart, and desperation ensues. Resulting in the over-use of make-up to compensate for the bodies now falling apart.
Basically, women peak earlier than men do when it comes to physical development and shape. They mature earlier, and wither earlier as well if they didnt actively take measures like watching food intake and working-out. Likewise men maturing somewhat later, and having to face the supposed testosterone decline somewhere around the 30's.
