For me slimness comes from being in shape, not wasting away and having no muscle tone (like many holywood stars on the red carpet have). As for youth, if you keep a certain muscle tone and are not overweight (I'm talking a normal body weight) you'll look younger because most older women don't exercise (My mother and all my aunts are prime examples, they won't exercise to save their lives and wonder why they gain weight even if they don't eat much!!). Now, even younger women eat too much and are less active, the obesity levels have shot way up in the last 25 years and I don't think its healthy or attractive at all.
It seems that people have liberated themselves from all social pressure and that has given them the greenlight to do whatever they want, including letting themselves go. In the suburban US it is particularly acute; suburban life is less public so there's less of a social pressure. I've never seen so many obese people waddling out of their cars as when I worked in Freemont California.
