I was having breakfast the other day with a friend who was giving dating advice to his son.
For purposes of his anonymity, I am going to call just call him "Son." My friend said, "Son, you're 18 years old, so I am going to tell you how you date.
What is illegal is if one is a minor/below the age of consent and there is intimacy. In most of Europe the age of consent is 16 and below 18 a minor.
What the situation is in the US seems to rather depend on the laws of the State in question.
Statutory rape is probably–no, definitely–the most controversial topic for you, our readers.