Can you date someone who is 18 if your 16?

It is not illegal to date, but it is illegal to have sexual contact.

Can you date someone who is under 18?

As long as you are aware of the risks, the best judge is you. There arent any laws around being in a non-sexual relationship where one person is under 18 and the other over. Once you turn 16 its not illegal for someone to have sex with you no matter how old they are.

Can an 18 year old date a 17 year old if they were dating before?

Any sexual activity with a person under 18, even if both are under 18, is illegal. There is no exception for sexual relationships hat began before one of the parties was 18. Its still illegal