They are just dating, not getting married or anything permanent, right? I wouldn't worry yourself about it. If they are enjoying each other's company, that's fine.

In a few years, that age gap won't seem as strange. My mom was 4 years older than my Dad, and my friend is 4 years older than her husband of many years - they started dating when he was 16, and she was in college, and 20+ years later and three beautiful daughters, they are still happily together. They didn't get married until after he graduated from college, but now that is a long time ago.