Right, like what's the point in excluding this data? I guess it's probably safe to assume if you had sex under 18 you probably have had sex after 18 but still odd that it's like that.
I'm willing to bet that they just didn't want the legal hastle of collecting data on minors. Also minors are going to be more likely to lie about any potential sexual history.
As far a why they excluded man on man action I have no idea. If the origional graph was in an article about declining birth rates or something then that may explain it but idk what the original article was so idk if that is the case.