Comment by brewdad

The job requires 20/20 uncorrected vision near and far. After 50, that starts to become pretty darn rare. The mental load required to keep all of these moving parts in your head and recall both what you have instructed them to do and what needs to still be instructed in the very near future requires a mental acuity that is not a young person's game. It can also take years for an ATC to have the experience required to meet the most pressing needs. I can see why they don't want to waste their efforts on people likely to be unqualified soon after reaching a useful level of experience.