You need to be careful if she is at Uni. If she's smart she may take a lot longer to emotionally cripple. I'd recommend bit by bit degrading the worth of whatever she is studying and what she wants to do as a career. Just laugh softly when she starts talking about it and then when she asks why...
Go for it I say. Start by working on the mother. How much you care about her daughter and want to take things slowly etc. Spend some time round their house and over a few months you can work on convincing her that the age gap isn't important and that you and her daughter are suited. Think about...