I would be slightly concerned if it were my DD, TBH.
Is he a postgrad in her subject? If so, depending on what stage he's at, he could end up teaching undergraduates. It would not be appropriate for them to have a relationship in that situation, and personally, I think if it's a possibility, it'd make things very uncomfortable.
I have known someone at Cambridge who ended up dating one of the postgrads who was teaching in her subject, and at the time we all thought it was very cool (for perspective, her director of studies was dating another undergraduate student, which I bloody hope wouldn't happen today and shouldn't have happened then!). But, in retrospect, it was actually quite damaging and had a pretty horrible impact on her academic and personal life.
I don't think the age discrepancy is unthinkable, but I think dating someone who is, basically, senior to you while you're in a very academically pressured environment, is a recipe for issues. He's always going to seem smarter because he's further on; if they are in the same or similar subjects, unpleasant people are likely to gossip about whether he's helping her with her work. And I must admit, I would want to be very sure that he wasn't just enjoying her being the young, less-knowledgeable person. That's not a recipe for a healthy relationship.
Not saying it couldn't ever work, but those would be my reservations if it were my daughter.