An agent can't take anything from a player until they sign a pro contract. Doesn't stop loads of them playing young footballer lottery.
In my opinion the lads should stay well clear and take advice from the PFA if they're in an academy. The PFA will even negotiate a 1st or 2nd contract for a small fee.
Unfortunately coaches and agents don't leave it alone. From 16yo (and younger) young lads and their parents get approached and most get signed up. Promise of 'free' boots, event tickets generally persuade them to sign up.
I'm aware of one young local lad who has been approached by around 10 agents in the last year. His parents are having none of it. The kid is nowhere near getting offered a contract by his club.
As you say most will drop out by the time they're 20. When they do, the agent disappears into the sunset.