SOURCES:
A youth’s brain is still under construction until the early 20s. Weed messes with your memory and even the way your brain is shaped.
https://americanaddictioncenters.org/marijuana-rehab/long-term-effects/
https://youtu.be/bU8sNyANFW8

Regular marijuana use by youth may contribute to losing 3 to 6 IQ points; heavy and frequent use can lower the IQ by 8 points (the difference between being a manager and having a minimum wage job).  
https://www.apa.org/monitor/2015/11/marijuana-brain.aspx
https://www.drugabuse.gov/news-events/nida-notes/2013/08/early-onset-regular-cannabis-use-linked-to-iq-decline

Studies link heavy use to lower life satisfaction, poor friendships, and crime. It negatively impacts school achievement, and the chance of getting a good job, earning good money, and having good relationships.
https://www.drugabuse.gov/publications/research-reports/marijuana/what-are-marijuanas-long-term-effects-brain
https://www.drugabuse.gov/publications/research-reports/marijuana/how-does-marijuana-use-affect-school-work-social-life