At a barn I worked at as a teenager, the BO didn't care what time you came, as long as the gate was shut when you left. So me and my friends would be at the barn at midnight, playing with the horses
But I think it's because she knew and trusted us, I'm sure she wouldn't have been as comfortable with other people being there.
I think it's reasonable to have hours like 7am to 9pm, and having a sort of clause where if you arrived before 9pm, you could still stay later as long as you closed up the barn when you left. No showing up after 9pm though!