In Canada, young people can be held responsible for a crime as of age 12. So, police can arrest a teenager if they think that the teen committed a crime (for example, theft, assault, drug possession or trafficking).
But the law treats teens under 18 differently than adults.
In some cases, a teenager might get an extrajudicial measure or an extrajudicial sanction. If so, the teen might not have to go through the usual court process. In other cases, a teenager might be officially accused of a crime and have to go to court.
Responsible before age 12?
A young person can do something wrong, but it is not always a crime. An example is breaking a window while playing baseball. Young people can be held responsible for these acts even if they are under 12 years old. But, in these situations, we are talking about civil responsibility, not criminal responsibility.