I'm one of those teenagers that practically runs a barn! Have been doing so for about a year now. Haven't gotten any complaints yet.
5-8 years of experience? Sure. Absolutely. Teenagers usually able to perform simple tasks like feeding, leading from pasture to pasture, blanketing, and turning out in proper places at proper times. Smarter breed than most give 'em credit for! Very good for stall cleaning. Somewhat poor attention spans, but can't have it all.
I've met a couple teenager "barn managers" who I'd board with in a heart beat.
As far as how young they could be... 16 would be my minimum age to work unsupervised.