Not that Wikipedia is the source for accurate info, but according to this listing, pantyhose showed up earlier than that:
Pantyhose - Wikipedia, the free encyclopedia. I'd think pantyhose were probably more readily available to a woman working at a Manhattan ad firm than to a woman in the midwest (a la Peggy Sue) at the time.
Matthew Weiner is SO meticulous about that sort of thing. I'm sure he researched it before he included that scene. Check out the article in the NY Times Magazine from a month of two back. :up
I thought there were TWO little ones at the house - was it Peggy's mother's house? - when Peggy went there for dinner. One called her Aunt Peggy, but there was another child there too and that's the one she went in to say goodnight to. Am I imagining that? :shrug