Rachel McAdams could easily play Jean if Meryl Streep, Helen Mirren, or Jane Fonda play Violet. She is that young-looking. It has nothing to do with her abilities, but when you are casting for the screen you always have to consider an actor's persona and looks. It's not whether you can make them look younger or whatnot, she just has made a career of playing younger roles (and she does it very convincingly). The line that Violet tells her during the dinner scene that goes something to the effect of "Women are ugly when they get older...as you're starting to find out yourself," would make absolutely no sense and would fail to be any kind of funny if McAdams played Karen.
Now, Somethingwicked's suggestions of Debra Messing and Christina Applegate would be ideal casting.
Has Helen Mirren played an American on film before? I know very little about her career and have only seen her play her very British roles in THE QUEEN and GOSFORD PARK.
I really would love to see Streep sink her teeth into Vi, and see what kind of sparks she could create in her scenes with whoever plays Barbara (hopefully someone like Rachel Griffith, Toni Collette, Laura Linney, Kristen Johnston, Patricia Clarkson)
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"