There are two things. Is it surprising? Not really. We know that cannabis is among the first substances used when someone starts using drugs, with tobacco and also alcohol. We know that cannabis will be one of the first drugs used by youth in general. We also know that the peak from an epidemiological standpoint, although I'm not an epidemiologist, is indeed during adolescence or early adulthood; so it's not very surprising.
Is it of concern? As I said, the younger the exposure the more likely someone is to have long-term consequences from cannabis exposure. Obviously, if that peak occurs later in life during adulthood when the brain is well developed, more solid, and less flexible, we might think that the burden associated with cannabis exposure might decrease.
Yes, it is concerning that it occurs at a young age, rather than exposure occurring later.