Why do teenage boys wear their trousers low and baggy?

That would be to allow for comfort in the leg and groin area. Males often find the urge to run, jump, or do other such activities, and often baggier pants allow for breathing room, as well as maneuverability.

There is also a note I should make: Boxers over briefs, loose over tight. Tight clothing increased the temperature of the body because it holds heat into the body. This also constricts the flow of blood slightly to areas within that radius. This can lead to discomfort and a lowered sperm count.

There may be many underlying reasons, but one can only speculate on it.