It’s also the only industry that is legally allowed to practice ageism. You have to start before or up to 31 years of age. You’re out at age 56. This figures into how the benefits are structured.
Certainly not all departments, but many fire departments have an upper hiring limit for new hires. Above that age you can only be hired as a "lateral" (transfer hire from another department).
You can still do contract ATC work after 56.