I agree with Brighteyse. Although, I think you should at least be 21 before you are able to be the barn manager, and that to me is almost too young for some.
I started doing 99% of the work at a barn unsupervised when I was 16.
It was up to me to do all the care on Saturdays and Sundays, morning and night.
I had 40+ horses to feed then move from stall to pasture while cleaning all the 20-30 stalls both morning and night.
I also worked 20+ of them, sometime both days.
I also was responsible for all the water troughs, cleaning and filling.
I also had to bath 20+ each weekend during the summer.
Most days I would get there around 6 am and leave around 1 pm, go home eat lunch take a nap and do it all over again at 4pm. Most nights I was out of there by 9pm, but there where the rare nights that it was more around 11pm.
Luckily they had golf carts so those came in handy. I also would bring a buddy along from time to time, sometimes a horsey friend that would help but 9 times out of 10 they were non horse people there just to talk to.
I think the most annoying thing about it was the random boarders that would show up and add another thing to do to my list or just get in my way and cause me to run behind schedule.
But all in all I loved that job, I did it for about 3 1/2 years, not counting the 6 years before when I did it all for free (of course then I was not by myself).
I would almost take the pay cut and go from my now Emergency 911 Dispatching job back to being a stable hand, but I love my current job a little more :)