Not sure how relevant this is though tbh.
Maybe for your average academy kid like Wilshere or Iwobi who's in the system at a decent club from under 14, getting the right coaching and the right experience and being moulded into a professional footballer then 18/19/20 is a significant age to assess them.
But when you're working at a full time job and only training two evenings a week with a 7th tier club? I doubt the same boundaries apply. It might take him until he's 23 to be at the level of what we'd expect of a 20/21 year old but all that doesn't mean he couldn't get there. All it'll mean is his career as a professional will be a few years shorter than some others.