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. But the situation you describe is not illegal.
Is it illegal for a 16 and 18 year old girlfriend?
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.
Does age of consent include kissing?
The age of consent is the age at which a young person can legally agree to sexual activity. Age of consent laws apply to all forms of sexual activity, ranging from kissing and fondling to sexual intercourse. All sexual activity without consent is a criminal offence, regardless of age.