On reflection, I think Auntie Shan might be onto something here.
I remember reading Stephen Jay Gould a couple of years ago, and he said that humans are pretty much juvenile apes: we don't mature in the same way as other apes and our face shapes tend to be much more child-like (this is called 'neoteny', if you're curious).
In terms of natural selection, the theory is that the younger a female looks, the healthier she looks for breeding purposes... because studies have shown that straight men prefer women whose faces look more child-like, whereas straight women prefer men whose faces are less child-like. This might be tied into the fact that eggs tend to deteriorate from age 40 onwards, whereas sperm is pretty much viable at any age.
So it could be that women, who have a shorter fertility span, have softer, more 'juvenile' features due to natural selection, and that these features are considered beautiful because they're desirable for reproductive purposes.