Lewington handed Young his first pro deal while at the Hornets.
Young will now join Manchester United and attempt to become a legend at one of the world's biggest clubs.
And, fittingly, Watford will land around 15 per cent of his transfer fee.
Lewington told the People: "It's so pleasing when a lad like Ashley makes it to the top.
"He's a terrific talent, one of the best I have ever worked with, and I'm sure he will deal with this move and do really well for United."