I see older very skinny women who look old and frail due to being very skinny. You need some fat under that skin to plump it up to look youthful. Catherine Deneuve I believe was the one that said that after the age of 40 you have to chose between your face and your ass. I say all of this as a 40 something woman.

Not fat, you need muscle mass, that's why some older people hunch over.

Harder to gain weight. Getting called skinny, etc.