I am not sure that dating has ever been legally defined. If anyone offended you, the blame is ultimately mine and I apologize.
As I also pointed out in my initial respsone to you, I also have never seen a specific law which states such, as an example: "It shall be against the public policy of X state where a person over the age of 18 holds out that he or she is engaged in human conduct or interaction that is commonly and socially known as "dating" with a person under X age.
As an extreme example, I have a cousin who is 22 and she's dating a guy who is 46 years old.
They have their own place, get on great, and they've been together about 2 years.
Obviously there would be problems with an 18 year old and a 12 year old, but that does not create a statute I can quote her, or you.