Because that, in itself, has no real bearing on the plot. It really doesn't matter if she's older - after all, even in the film, she's the one that took Vivian in and showed her the ropes.
As to the changes, I applaud the attempt. I find watching the film now is pretty cringe-worthy with its old fashioned tone - being nothing without a man. I do think they are only expanding on what DOES exist in the film. Vivian is more headstrong, more responsible than Kit, not involved in drugs, etc. Also, when he offers to set her up as a "kept woman", she turns him down and makes plans on her own future. If they can pull more of that out of her, I welcome the changes.
That being said, I'm not terribly keen on seeing this, but it might have moved a notch or two up the ladder with this info. (Maybe.)
If we're not having fun, then why are we doing it?
These are DISCUSSION boards, not mutual admiration boards. Discussion only occurs when we are willing to hear what others are thinking, regardless of whether it is alignment to our own thoughts.