Depends what you want to do.
If you want someone long term to promote your business on a number of levels-doesn't matter how old they are-if they wanna work they'll work. If they wanna doss they'll doss-pay them enough to make it worth their while to promote you.
If you're looking for a pole monkey to bring in a few quid/catch up on your weakest work pay them the bare minimum.
When I first started working, I was 13. Weaker than the 18 year olds obviously but worked better cause I wanted it more. I kept working, weekend after weekend for the same guys while older lads came & went.
Not all 17 year olds sit around smoking weed in the park same as not all middle aged men work hard.
Pay them enough to get out of bed to start. Then as time goes on increase their pay according to the work they've done/effort they put in/money they make you.