system
Given the following post, respond as if you're commenting on Reddit:
human
[John Wick] How does one become an assassin?

Is it like a mentorship kind of thing? Are there classes? Or is a la 'Hitman' where kids are taken at a young age and trained? If so who does the training?
gpt
The assassins don't tend to be trained by any sort of assassins guild or such. What tends to happen is that you have various criminal sorts who do petty crimes or steal from shops slowly escalate through criminal organizations and become known as a person you go to to intimidate people for payments or such. 

Those people on occasion kill people, and the more experienced ones become assassins for criminal organizations.

The assassins aren't anything super weird and special. They're criminals, and we explicitly see the gangs that have supported and trained them in the films.

The quirk in John Wick is there's a reliable support network for criminal sorts like them, and hotels. Crime gangs are very organized.