I am the manager of a concrete pumping business. It doesn’t really affect bodybuilding or vice versa. I never went to college.
You don’t need to be focusing on specific body parts until you’re relatively experienced and have a decent amount of strength and general size to you.
Bodyweight exercises and cardio is all I can really recommend to you if you aren’t interested in anything else.
There really isn’t an “ideal” age but 12-13 is probably the best time to start lifting casually. I’ve seen 12 and 13 year olds that are stronger than a lot of guys my age simply because they grasp the concept and the dedication so early on.
I’m just going to keep this simple: no.
Shedding water weight healthily is easy enough. Diuretics are unnecessary.