Laura Benanti is the best "name" actress for the role. She's got the comedic and dramatic chops, and Lord knows she has the voice. Her age is irrelevant-- Julie Andrews originated the role in the musical at the age of 20, but she's the exception to the rule. As others have noted, Shaw's original Eliza and Audrey Hepburn were more than a decade older than the character they played. Plus, a more mature Eliza might help offset the icky factor that is the difference in Doolittle and Higgins' ages.
But, personally, I prefer casting a fresh face in the role of Eliza. It's a star making role, and casting Colin Firth as Higgins should be more than enough to encourage box office sales. Why not use the opportunity to introduce a new face to Broadway? And if this proposed revival is meant to a transfer of the Julie Andrews-helmed Sydney production, the involvement of the beloved star of the original should add extra punch to the box office. I must concede, I am an unfailing romantic, and day dream about director Julie Andrews creating a new star in her signature theatre role.
For the record, that Sydney production has cast Alex Jennings and Love Never Dies star Anna O'Byrne in the leading roles.