I was an acting machine tech/supervisor on shop floor at 19 only my second year of employment, and an (assistant) manager at 21. When I was assistant, the manager wanted me to handle allocation of staff, but only 3 weeks after I got promoted he quit his job, and wasnt immediately replaced, I had only done a few days of training and was thrown in the deep end. My agency staff supervisor was in her 40s, I delegated a lot of things to her, but I had to deal with non agency staff directly as she had no authority over them. There was a bit of a wtf from some people early on, but it wasnt that bad, which I think was because I was respected as a known hard worker on the shop floor rather than some young guy been directly recruited.
The biggest issue for me is that this job of assistant manager was kind of an experiment by the company, my salary was only about 20% higher than what I had as a normal shop floor operative as it wasnt a role that existed before and the other 3 shifts didnt have it, and lower than the technicians, actual front line managers were paid about 3-4x my salary. Once the manager quit I had the same responsibilities, including dealing with the crappy side of the job where you have to be an ****hole in cutting down manning, so the company can meet its profitability targets, this was an eventual toll as I was seen more and more as a manager and less as a hardworking shop floor dude. I kept been promised they would back date a proper contract but it never happened. On top of all this I was doing my management training during days I had to also work my shift when I was on nights. There was a spell I used one of the first aid rooms to sleep. Training ended at 5, shift started at 7, so had maybe an hour or so sleep if lucky.
I eventually got moved to a office role where I no longer had to be responsible for shop floor staff and my training was stopped then as well which in the end was a relief as I did find been a front line manager overwhelming.
To give an idea of the numbers I was responsible for about 60 people on my shift.