This almost looks like a staged photo for an older adult fitness magazine, if such a thing existed in the 1940s. Perhaps, though, it was just evidence of a fun beach day among family and friends.
Given the high-waisted pants, the short top, and the oxford shoes, I’m fairly sure this photo was taken in the mid to late forties. The pants look to be without pleats, pockets, or a belt; the short blouson style top apparently has a surplice structure for extra movement. I like to think she found common sewing patterns and then made alterations to make them more exercise friendly. The shoes look like plain white leather oxfords. Or do you think perhaps they are canvas?
Staged or not, this photo reminds us not to make sweeping generalizations about historical health and fashion trends. Some older women happily turned to pants in the 1940s, and those who paid attention to exercise are also nothing new.