When Jed and Granny see Elly May come down the stairs in her Gibson girl dress, Granny says that that is her sixty years ago. So at the turn of the century, Granny would have been around Elly's age, late teens/early twenties. Meaning Granny would have been born the 1880s and would now be in her seventies or eighties.
(Granny tells Jed that women's clothes today reveal too much and don't make men wonder)
Granny: You take them women down to the vittle store. Them women come in there wearing little shorts and something they call stretch pants. They all look like a four pound sausage stuffed into a two pound skin.
Jed: Right in the vittle store, huh?
Granny: Oh, it's shameful. Why, them women show more than a side show wiggle dancer at a county fair. Ain't you been down there? Ain't you seen em?
Jed: No, but you sure put me in a mood to go.
Jed: Does your head smart, boy?
Lafe: Answer the man.
Dub: Only when Pa hits me.