My brother was 15 when he began dating his first girlfriend, who was 19 upon them beginning to date. Neither one of his parents (we have different fathers, same mother) thought there was an issue with it. They dated for over a year. I on the other hand had a huge issue with it, and wondered what someone who was freshly an adult would want with someone back in high school? When you graduate high school, you enter a completely different stage of life. That would be like someone who just graduated college dating a 20 year old. The age difference isn't big year-wise, but life experience-wise, it is huge. Not to mention, 18 and 19 are virtually fully developed whereas 15 is in the middle of puberty. Legal or not, it is highly inappropriate and if that was my child I'd report the adult who was with them.