At the start, end, and throughout the film, the old woman narrator seems so obviously dubbed by Stanwyck that I think the part was played by an older actor or someone else in makeup.
It didn't look at all like Stanwyck to me in the old lady costume and make up.
01. There's just about nothing a good make-up artist can't do. Here's the listed make-up people for this flick:
Makeup Department
Robert Ewing ... makeup artist
Wally Westmore ... makeup artist
Charles Gemora ... special makeup (uncredited)
02. All things considered, he did an ace job. Stanwyck still had some youthful fill to her face. Compare her to her actual age in the Thornbirds, in which there isn't any baby-fat, but how people actually age. I don't believe she ever had plastics. She held up very well in spite of the smoking.
03. Perc & Wally Westmore had their own line of make-up, but nothing could hold up under intense camera lights like Max Factor.
04. Of course, today Gemora would be listed under 'Special Efx' & would have so many more resources to work with.