It’s also hard to hire for. Candidates for job openings must be between the ages of 21-31 years old. Yes they are legally forbidden from hiring anyone older.
Since they have a mandatory retirement age of 56 (if they're not retired earlier for health conditions) it's not crazy to have an age cutoff for intake. Why put someone through a 2 year training with a high failure rate if after they make it through all of that you'll get at most 10 years of work out of them?
https://www.faa.gov/air-traffic-controller-qualifications