VL; you asked about two 15 year olds having sex, is it a crime? That would depend. If it is consensual then No it isn’t a crime. If it’s not consensual (one forces the other) then it would be a crime. However, it would have to be proven in a court that it was forced sex, and that can be difficult.
The point of this law is mainly to protect kids under 16. Even if a child said to an adult of say 30 years old, “I want to have sex with you”. , and if the 30 year old did have sex with the child (under 16), then the 30 year old would be committing a crime! Before in Canada, it was only a crime if the child was under 14, or if it could be proven (which is tough to do) that the person of 30 years was in a “position of power” (i.e.; teacher, coach, parent, etc) over the person which was 14 or 15. 16 or over however was always fair game (which I personally don’t agree with), and the burden is on the prosecutor to prove it was forced (or unwanted) sex.
Hope that helps,