Because they were from the states that have it at 17 or 18 most likely, for example the most populous state, California, is 18. It's a common misconception that it is set at a national level at 18. Most are 16, some 17, and some 18. Some will call anything below the given age illegal outright, but some have "Romeo and Juliet" exemptions when both parties are close in age (for instance, allowing a 15 year old and a 16 year old)
