It is generally legal for a minor to be in a non-sexual dating relationship with an adult child. This includes the minor dating an 18-year-old, 19-year-old, 20-year-old, 21-year-old, etc. Though note that just because it is not unlawful to date someone underage does not mean that anything and everything in the relationship is legal. Some non-sexual conduct in the relationship may still be against the law.