A gap certainly, however both are legal adults (here in England anyway) and it is their decision and right to choose who they want to go for. No one else has any say. They may not approve, but a good friend or relation will keep those feelings aside and be happy for them and be there to pick up the pieces if things go wrong.
My younger sister who has just turned 20 is dating a guy the same age as our father, so i know hos frustrating it can be, however no one has the right to tell someone else (especially an adult) who to love and go out with.
However if it was an adult going out with a legal child, then i would certainly disaprove and have my opinion known.

I think it's fine.