Following up on my previous post, someone has posted two clips of Carol Bruce in her prime as a singer:
Her sultry voice seems to have gotten raspy rather more quickly than it should have; by Do I Hear a Waltz she sounded very different, even though she was in her 40s. Her early drop to an "older"-sounding voice may have contributed to the fact that she often seemed to play characters who were older than she actually was.
I'll also note that her casting as Julie in the 1946 revival of Show Boat -- the first revival of that show -- helped change the casting of the part. It had been written for Helen Morgan, a high soprano; after 1946 it's mostly been a part for lower-voiced singers like Bruce.