I don't think O'Hara reads too old for Eliza. Really, I think a role like Eliza, which has been essayed by so many people over the decades, should be approached the way that Shakespeare characters are approached and just cast people who'd be spectacular in the part with little regard to their age (within reason, naturally). I'm sure O'Hara would be great as Eliza Dolittle, but it's the type of role she can play in her sleep. I'd love to see her do something very different from THE KING & I next.
Laura Benanti is made for the role. The reason she'd be my top choice is that she can be such an exciting force on stage. She is hilarious and tough and then she slays you with this gorgeous soprano voice. She is one of the few actresses of her generation who could make you laugh out loud with her "Just You Wait," make you feel all the feels in "Wouldn't It Be Loverly?" and "I Could Have Danced All Night," and then be tough as nails in "Show Me." As long as they don't cast Kelsey Grammer, I don't care much about who plays Higgins, but I would buy my tickets in a heartbeat if Benanti is cast.
"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"