The Campers: 1930s | Shorpy Historic Photo Archive
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The girls' hair is definitely forward-looking (to the 1940s). My gut feeling though is that the older women look reasonably "with it." In other words they don't look like they'd be wearing styles dramatically out of date -- they'd simply not pick up on the latest change as it happens. Given that the car is '35, my expert opinion puts it at 39-40.
No one has mentioned the family dynamics of this photo: Mother, Aunt, Father, older daughter, younger daughter -- not the kind of campsite you put up and take down in a day, so probably has been set up in a campground for a week. The Uncle pulls out the camera on the day your leaving, and catches the youngest not wanting to leave the boy she met. Oldest daughter in a good mood 'cause they are going home, Mother tired of cooking and cleaning in the "rough," Father wanting to get home to see the game....