Based on the facts you have presented, it is not illegal for these two people to have sex as long as it is consensual. The age of consent in NJ is 16. As long as both parties are over 16 and some other factors not present in your question dont exist, there is nothing illegal about sex.

Can a 17 year old date a 20 year old in New Jersey?

Yes, it is legal for them to date. If they have consensual sexual intercourse, that is also legal, because he is over the age of consent in New Jersey (16).

Can a 23 year old date a 17 year old in NJ?

Dating is not a crime. Its the sex that gets you in trouble. However, the age of consent in NJ is 16. No legal advice is given here.

What happens if a 17 year old runs away in NJ?

Running away is not a criminal offense in New Jersey. However, a runaway may already be involved or may become involved with a law enforcement agency. Running away for more than twenty-four hours constitutes a juvenile-family crisis, N.J.S.A.