Looking at the history of the nudism lifestyle, it got introduced to the United States in the early 1930s and started to reach its peak in the 1950s.
So a middle-aged female nudist in the 1950s would have been born between the 1890s and the 1910s.
These are women who lived through The Depression, two World Wars, and had much less sophisticated medical care, OBGYN care, and fairly basic fitness and dieting knowledge, yet these women maintained their figures will into their later years.
That's in comparison to today where a nudist who was born in 1965, looks like an absolute slob.