Is it legal for a 16 year old and an 18 year to date?

The only illegal thing is if they were to have sex. That is illegal because the 16 year old could be under the legal age. However, it isn't illegal to date anyone, what ever age.

If you are just dating, and your parents have no objection there is nothing illegal here.
If you start having sex, then you should know the age of Consent in your state or country.

Read more about the Age of Consent in the Related Questions below.