No this is not illegal. Simply dating a person over age 18 is not illegal. It can become illegal for a person who is 18 once sex is involved.

Can a 16 year old date anyone over 18?

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.