Understood, but age alone won't give us the needed flavor the show requires. Still, if we're looking at 70ish in earnest, leave Jane Houdyshell out (she's younger than O'Hara, 59; surprising?) 70, B'way, comic singer, hmmm.
Mary Louise Wilson?
"I'm a comedian, but in my spare time, things bother me." Garry Shandling
Apart from the fact this is a role that should really go to a much older actress, Veanne Cox strikes me as the perfect Primrose.
My first choice would be - caveat: I have no idea if she'd be up for it, she's two years older than Mary Louise Wilson and they are both in their 80s - Patricia Routledge.
I haven't heard Anita Gillette sing in 40 years, but otherwise she's an inspired choice. She is also 78, however, and may not be up to what is a fairly physical role.
My memory of the original staging, at least, is that Mrs. Primrose does a lot of running around on moving set pieces.