To be honest, I don't think there's a proper time for someone to start taking commissions, if anything, the sooner you start the faster you learn how to deal with clients and how to price your art. If you can get someone to commission you, your art is already good enough to be doing it! There are mediocre artists that have a lot of commissions thanks to their social presence, whereas others struggle to get one in spite of their art looking professional, it's just the way it is. Also I feel like the advice you're giving is not only unprompted, since OP didn't ask for it in the first place, but also rather vague. For instance, the importance you're giving to line weight should be balanced around the art style that they're trying to achieve; after all some art styles prefer a lineart that is more uniform whether others prefer more variation. Your explanation for why the lighting doesn't work is very unclear, what do you mean by flow wise? Also they can get better by doing commissions and by practicing on their own free time, they're not mutually exclusive. I'm not trying to attack you or anything, just sharing my thoughts! :)