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 18 year old female date a 16 year old male?

If there is no sexual contact and there are no nude images exchanged, an 18 year old can date a 16 year old. The risk is that something sexual in nature would occur and/or you would receive an image on your phone of the 16 year olds body