It is not illegal for an adult to date another adult. Further, even if one of the people is under the age of 18 they can date as long as their parents consent to the dating.

Can a 21 year old date a 18 year old?

As long as both parties are 18 years of age or older, not being forced, not unconscious, not too drunk to consent, not mentally impaired to the point they cannot consent yes, an 18 year old and a 21 year old can have sex.