Although not Grizabella-like, she did a fantastic job with Maureen in the Hollywood Bowl RENT a few years ago. And the Cats costumes & make-up could transform anyone, so her looks don't matter.
I don't mind. In fact, I like. Her voice is pretty good and she can pretty much belt the living crap out of it. She did very well in Rent. And Grizabella isn't exactly Lady Macbeth. She's on the stage for a total 20 minutes, mainly singing the same song twice. I say yes.
Laurie Beechman was only 30 when she played the role in the first national tour and replaced Betty Buckley shortly thereafter in the Broadway company. Elaine Paige and Betty Buckley were both also in their mid 30s when they opened the London and Broadway productions. I'm guessing that if you went through the list of replacements you wouldn't find very many who were as old as Judi Dench was when she left the original production (who would have been a few years shy of 50 still). As others have stated, the age of the actress doesn't matter, since she'll be wearing five pounds of make up.