I think the contrast between Stone and Midler is exactly what the two roles need.
Of course Stone is actually about 12 years younger than Midler, when the characters are supposed to be the same age. To contrast, Michelle Lee is only 5 years younger than Linda Lavin and Marilu Henner is only 4 years younger than Rhea Perlman.
Midler could probably pass for 60ish, even on film (and especially with Hollywood magic), so the difference probably doesn't matter all that much.
