I have been thinking about this since yesterday.
When I was young (till 25) I was tall, blonde, slim (size 10-12), modest norkage. Pretty. I admit this when I look at pictures of myself, I did not think I was pretty back then (like in Oenophile's post).
It is hard coming to terms with weight gain when you have spent your entire childhood, teen age years and young womanhood as a slim person.
It might be equally hard for somebody who has been beautiful all her life to "lose her looks".
But I think this is much connected not just to your own vanity or lack thereof, but also your own perception of beauty. Beauty is not only reserved for youth. Some women age extremely gracefully, and you can see that they have been, and still are beautiful. Glenn Close in my opinion is one of them. Sharon Stone another. If you think that only youth, or youthfullness is beautiful, you might have a hard time adapting and finding beauty in the looks of an aging person. Or indeed you may mourn the loss of your own beauty.
Why do some think George Clooney, or Sean Connery look even better now? Ralph Fiennes? (It is easier to see beauty in a male aging face than a female, so apologies if I cannot think of that many old beautiful women!)
There is beauty in maturity, beauty in life experiences, beauty in a weather beaten mature face.