I will tell you what I would tell my own child:
Cannabis, like any recreational drug, steals the most precious resource we have: our time. I've been on benders in my life that I simply can not remember what I did during that period, sure, but that's not what I'm talking about.
What I am talking about is the time any person needs to become a well adjusted, functioning adult. When you are taking a recreational drug, you are not dealing with life, you are on a 'get away'.
Life is the best teacher and recreational drugs are how you forfeit that precious, amazing, education.
You don't have to go far to see this in action. Think of someone who is a habitual drug user and ask yourself 'are they emotionally mature?'. They aren't. They are taking an escape. Recreational drugs are antithetical to emotional growth.
This type of behavior is ESPECIALLY harmful if you are taking recreational drugs early in life. It simply puts you behind.
I wasted 20 years of my life 'partying' even on a casual basis. Time slips away and being high puts time on a damn bullet train.
I heard recently that the fastest way for an addict to relapse is a windfall of money. Money doesn't help them get out of their situation because money is hard to manage, particularly if you are emotionally immature.
tl/dr: Recreational drugs make you immature which will prevent you from going very far in life.