I believe that mentally, 45 and 20 year olds are in two vastly different places. A 20yr old should be allowed to experience more of life, socially, educationally, career-wise. Her way of thinking and assessing life's lessons/moments/experiences hasn't fully evolved. She's still forming opinions and maturing. It doesn't matter if she appears more mature than her age, I believe she should still be given the opportunity to develop more, and that takes time.

Plus, when she reaches the stage where she's preoccupied with finding/establishing her role in the wide, wide world, he's probably already at the age where he's done with all that and wanting to settle. So as the two grow older, she may end up feeling saddled with a senior. I'm not saying a relationship like this can never work. I'm just saying it requires some serious introspection on both sides. What stages of life are the two at now and what are their goals for the future? If they are very different then trouble may be ahead.