Surely you understand that there is a difference between the production red blood cells, which are the same at birth and elder age. Versus: The development of the prefrontal cortex which is responsible for processing and adapting one's thinking in order to focus, control how you behave, and make (smart) decisions.

Which almost all drugs directly interact with.

Puberty is where your brain learns to find its baselines, to discover who you are. Using drugs from a young age impacts who you might become.

Now the difference between 18 and 24 isn’t going to be that big and amounts matter. 1 joint at 15 isnt going to harm you more then then alcohol at the same age. But if your baseline is to be impaired all day, thats going to have an impact.