I am 17 years old my boyfriend is 20 is that a legal age diffrence between us.

It doesn't matter if you are just dating. There are no laws that set a legal age difference for one to date. The laws all refer to sexual activity between you.

Those laws vary from state to state. To find the law for your state just try googling Age of Consent state.

At your ages you are on the edge of a viable age difference. I really don't see a big issue in you dating him as long as your parents approve and there is no sexual activity. In all places once you are reached 18 you will be above the age of consent.

As noted, a lot will depend on where you live, and if you are having sex.