For whatever reason, one doesn't hear much about Desmond Morris these days, but I recall an interesting program he did a few years ago on human sexuality (and The Naked Ape was a favorite book of mine when I was a teenager and there weren't very many books available viewing humans through a zoological lens). I seem to recall him describing humans as the sexy ape, both behaviorally and in terms of secondary sexual characteristics, and he also emphasized the importance of high levels of bilateral symmetry in typical human conceptions of beauty.

And while yes, many individuals can appreciate the particular forms of beauty that come with age and experience, I think the general human bias toward beauty standards based on youth and good health make biological sense. It seems like simple common sense that indicators of fertility--like post-pubescent youth--and indicators of the ability to successfully care for children--like good physical and emotional health--would correlate highly with what most humans find beautiful and sexy.