I was watching the film version of Evita earlier and marveled at how young they made Madonna look during the scenes where she's supposed to be a 15 year-old.
Then the camera closed in during "Eva Beware of the City" and the Material Girl's 30-something reality shattered the illusion for a sec, but only whenever she'd make any strong expressions--her neck gets all wobbly and loose, making Eva appear to momentarily age 25+ years. Ah, the magic of Hollywood.
Like someone else said, Babs will be OK because nobody cares about Gypsy Rose Lee's momma. And I generally don't think being totally accurate is that important when it comes to age with the vast majority of roles. There's a reason nobody flinched when Madonna appeared as a 15-year old Eva on screen...nobody cares about the real Eva because most didn't know who she is, and even if they did, it's not something they feel passionate enough about to raise a stink over. Leave that to people like me, and even I don't care.
Now if Babs was cast as a 15 year old, then we'd have a slight problem. Even then, the theatre lover in me would at least find it worth investing some suspension of disbelief for the screen, if only for someone like Babs Streisand.
Recreation of original John Cameron orchestration to "On My Own" by yours truly. Click player below to hear.