roxy, a 21 year old girl is old enough to make her own decisions but think of it this way, she would be dating a man who's in the same age range as her dad. That may be ok for now but as the years go by and they get older, many things will become more difficult. Marriage is hard enough as is without adding a generation gap to the equation. If it was possible to just have fun for a while knowing you will move on I'd say "go for it" but if you're thinking long term I'd advise against it.
It's not exactly the same thing but when I was 20 I had a relationship with a 37 year old woman with twin daughters who were 19. We had a great time but within a year I had met a girl my own age and we agreed it was time for us to move on. Our age difference made it impossible to think in permant terms and the single biggest reason was having kids. There was no way she was going thru another pregnancy while I knew I would some day want a family so that made it easier.
She's got a big decision to make so best wishes to her whatever she decides to do.
