If you shave your face early, (I'm a 16 year old male) can it stop body growth or reduce the amount of time your body has to grow? (Perhaps your shaving signifies you going into adulthood and thus your body believes it no longer needs to grow)
Facial hair doesn't come in evenly. Usually the muzzle area is first, upper lip and chin, this has the greatest blood circulation, which nourishes the developing hair follicles.
You start shaving if and when the hair doesn't appeal to you.
I didn't mean to suggest you shouldn't shave at 16, I'm sure a lot of young men shave at 16 and younger. You can't put an age on it, my brother started shaving at 24 and then he didn't really need to. You go ahead and shave when you want to.