Age 31 limit is because of age 56 mandatory retirement. Allows for 25 years of service for retirement purposes
Very few people make it to 56. There are some spectacular controllers at that age, but most are on the downhill side of control judgement and decision making skills. And the medical qualifications get harder and harder to maintain as the years of sedentary job conditions pile on. Most controllers become burned out by 25 years of shift work and being a government employee. Thats generally what forces them out the door. And the anecdotal controller that drops dead right after retirement. It happens, so most guys aim to get out as soon as possible to re-acclimate to normal life and normal sleep patterns.
Obviously that doesn’t fit with the popular idea that there are grizzled, old controllers like Lloyd Bridges in Airplane. I think he was in his late 60s. For some reason I thought maybe Fred Thompson was older, but he was in his late 40s when he worked on Die Hard 2. But he certainly seemed like the old guy who has been on the job forever.