The 18 year old can get in trouble because he is dealing with a minor.
In one case, a 17-year-old was sentenced to 10 years in state prison for having oral sex with a 15-year-old. Therefore, it is safest legally and socially for an adult to date only other adults.
At 18, your teen can vote, buy a house or wed his high school sweetheart.
In some societies someone in their late teens dating someone in their early teens would be frowned upon, in others it would be quite acceptable, even encouraged.
It is necessary to conform to the ethics of the society in which we live, lest we offend the moral police.