If the brain's endocannabinoid system is constantly disrupted with cannabis while developing it can affect mood, motivation, learning and how memory circuits get wired. Short-term memory loss is a common side effect of long-term cannabis use because it can cause less information to get saved as memories. Cannabis can reduce your ability to pay attention and organize yourself along with affecting your everyday mood; it can bring on prolonged anxiety and depression.
According to a study done at the University of Waterloo, researchers found that "The odds of developing any drug abuse symptoms by age 28 were non-significant if cannabis use had its onset at ages 15 to 17, but were significant and almost doubled each year if onset was before age 15. Even if those who start smoking cannabis at 17 years were at lower risk, frequent users (20 or more times a year) at age 17 had almost double the chance of abuse by age 28 than occasional users."
The following video talks about how a teenage brain develops and how early use of cannabis affects that development along with the short-term and long-term effects.
Sources: Science Daily