Depending on what state you live in, it's likely that it's legally fishy.
However, I would have a problem with it ethically because a young man in his 20's is in a completely different life place than a young woman who is 16.
Now I know that some young people will say, "I'm very mature for my age," and that might be true. But maturity and life development aren't the same. A mature 16-year-old girl cannot understand what it's like to be a 50-year-old menopausal woman because she is simply lacking the life experience to get it. That's not a slam on anyone's abilities or intelligence.
Additionally, I'd be curious about why a 20-something young man is dating a 16-year-old and not a woman in his own peer group. When you get to age 22, you'll look back at your 16-year-old self and get how kind of slimey it seems for men that age to pursue very young women.
There is a trend in our culture for men to pursue very young women. There are lots of reasons why this trend exists. Part of it has to do with the pornification of the larger culture and the sexualization of younger and younger people.
Honestly, I would have some serious concerns. Additionally, I think you should maybe try to discuss this with your parents. I imagine they would be concerned about their 16-year-old daughter dating a man in his 20s.