Honestly I can see how your dad would be bothered by it. When one of my bff's was 20 she started dating a 30 year old and the age difference was huge. No one could understand what a 30 year old would want with a 20 year old. In the end he ended up being a pyscho and a year after they broke up still stalks her.
Now on a positive side my other bff is dating someone 10 years older than her now and things are going really well.
I think it all depends on the person, but I'm always a little sketched out about huge age difference when people arent even able to legally drink yet. The lives of people at the college age and the late 20's can just be very very different.
I wish you the best with everything, hopefully it will all work out lovely and your dad will come around. It will just take time.