This fraught relationship dates back to Lane Bryant's maternity designs, which found a market among "stout" women, especially those over fifty. Similarly, junior sizes, once designed for the slim, high-busted figures of women in their teens and early 20s, became the size range of choice for women of any age who retained their "girlish" figures. (Thus "A size, not an age", the theme of a 1957 fashion show at Saks Fifth Avenue.)
I will discuss this quandary from the point of view of women over fifty, who must search for "age appropriate" clothing based on their body type.