If he's a good guy then it's okay. But be careful. For a 20 years old guy to choose someone younger sometimes because he have this strong domain, which ended up bad for you. Bullies, taking control, those kind of stuff.. Make sure to always stay on your toes. There's a saying "better safe than sorry", "it's better to prevent than cure", and so on.. Anyway, you know what's good for you.
I don't think it wise, but not wrong. Also, not illegal to date; only if you sleep together. Even if you did sleep together, which I don't reccomend; your parents would have to charge him with rape. If it was consencual some states would charge either both of you or neither.