The biggest problems are do the two of you share chemistry and common interests, does the attraction go both ways, and will the older guy be accepted by you family and friends. Maturity is also a factor, if the girl is just convinced she likes him without realizing some of the things mentioned above aren't true then she could be getting manipulated into it by a more experienced person, But if she's mindful of that and can objectively tell if she's risking anything then sure it's possible.
i 100% agree with what Orion said because it's true. I'm in the same situation, I'm 20 and he's 31. we connect because despite the age difference we have the same common interests, and I'm on the same level with him intellectually and that's important. I could go on and on to help you out but if you need to you can always message me. I hope everything works out.