Not to argue against weed, but it's actually bad for some people who have a certain genetic predisposition.

For some, it can be harmful regardless of whether they have good supervision or they need to learn how to handle the high.

It is known to exacerbate schizophrenia symptoms and hence is not recommended to people with those tendencies.

Edit: I don't remember the source, I remember this from the kurzegesagt videos on it.

https://www.youtube.com/watch?v=kP15q815Saw&t=17 (This is an early video, they changed their opinions later.)

Their more recent video - https://www.youtube.com/watch?v=qBRaI0ZeAf8&t=8 (This is a more recent video and is based on more concrete science. They're a lot more optimistic in this one).