¶ … Beauty of Youth The fact that physical beauty is so obviously apparent that we might never really bother to think about why that is true, or what it is about youth, exactly, that corresponds to physical beauty. It manifests itself directly in many ways, including the age-dependent differences in physique, body composition, skin, hair, nails, and teeth. Youth also corresponds to characteristic functional differences in eyesight, hearing, and vocal tone that affect others' perceptions about our physical attractiveness indirectly.

During adolescence and early adulthood, we experience a tremendous increase in our secretion of the sex hormones: testosterone for males and estrogen for females. Those hormones trigger muscle growth and other secondary sex characteristics in males and accentuate the curves on the female physique in conjunction with the other secondary sex characteristics in females, respectively. Another aspect of sex-hormone influence manifests itself in the characteristic differences in vocal tone between males and females. Potential sexual partners are more attracted to the deepest and strongest tones of male voices and to the highest and most melodic sounds of female voices, respectively. As we age, our voices become weaker and their tone and tonal range obviously shift in ways that make it relatively easy to guess a person's age just by hearing his or her voice. These stages of life also coincide with our highest relative fertility and virility, with females becoming pregnant more easily than...

That is because of one of the universal principles of the psychobiological evolution of complex organisms: sexual selection. Specifically, during the evolutionary period of any higher animals that reproduce sexually, reproductive "success" in evolutionary terms, is (obviously) substantially a function of physical attractiveness to the opposite gender. The more attractive the individual, the more potential sexual partners that individual will have. Moreover, in species that mate or "pair bond" for life, sexually attractive individuals are likely to attract more attractive partners, simply because they have more opportunities to pick and choose from interested opposite-sex candidates.
Because individuals (both males and females) who were attracted to younger-looking partners were simultaneously selecting partners whose body type corresponds to high fertility and virility, they were more likely to leave more offspring than individuals who were attracted to older-looking partners whose body type corresponds to lower fertility and virility rates. Over time, any such advantages in mate selection (and access to more short-term sexual partners before, during, and after pair bonding with a primary partner) would result in…