Personally, I believe that the aesthetic of a woman's slimness is essentially a "girl's beauty" or the pursuit of a girl's sense of humor. The basal metabolic value of a human being decreases with age, and older women or men are more likely to gain weight than teenagers.
For example, when I was in junior high school, I was 167cm tall, and my weight stayed around 55kg all year round until I graduated from high school, with no intentional dieting or exercise, and I remembered that at that time, if a girl had 60kg, she would be regarded as a fat person by her classmates. Now I am 26 years old, I feel that I am obviously easier to put on weight than in my teenage years, with exercise and diet control, my weight is still more than seven or eight years ago, ten pounds. And those forty or fifty years old square dance mothers, exercise than young people, but the weight of 130 pounds per capita up. The better conditioned women, even in middle age, will try to maintain a slim figure, the biggest purpose is to show young.
Similar aesthetics are also used by women to get rid of wrinkles, do apple muscle filler (face surgery), wear Lolita dress up and jk uniforms, in order to mold themselves into a cute "pseudo-girls", of course, with the accompanying body must be thin, will be like a young girl.
As for why women are young and beautiful, this may have something to do with the value of childbearing, which is more difficult after 35 years of age. A man marrying a beautiful 20 year old means he is likely to have more offspring than men of the same age who marry 30 year old women.
Given that humans are class-based, a significant other who is well off will bring more to the table than one who is good at manual labor. So often the popular aesthetic is "aristocratic" rather than grounded. Similarly, a woman's footbinding means she is from a good family and doesn't have to work on the ground, a woman's white skin means she is from a good family and doesn't have to work on the ground, a woman's melon-faced face means she eats fine food and doesn't have to chew, and a woman's thinness of course means she is from a good family and doesn't have to work on the ground. This is also the reason why white and thin beauty is more attractive to men than strong and dark peasant women.