That said the point is valid. Retiring people at a set age is illogical, dull and unfair. The military love fitness tests and health checks....use them with a bit of creativity. If a 57 year old is fit enough to carry out the full range of his military tasks, he/she should not be forcibly retired on age grounds.
It's not just the military, though. Civilian pilots are required to retire from public transport at the age of 60 irrespective of whether they can maintain Class 1 medical standards or not, and despite "pensionable age" being increased.
At least the military can and do use their discretion to keep someone in employment.