Well, in the original novel, Mame was in her late forties, iirc, which fits the story logic of being an aunt to a young boy. Rosalind Russell was in her forties when she played Mame in the stage adaptation in the 50's. Angela Lansbury was also just past 40 when she played Mame in the musical adaptation in the 60's. So I'd think casting a 75 year old would be a cockamamie idea. Ha. Then again, Meryl Streep could absolutely nail it, couldn't she?
I would look for someone fortyish or fiftyish with a Roz Russell look.