it's crazy because most law enforcement agencies for example...want you out after the age of 60. Doesn't matter if it's the FBI, the NYPD or some podunk sherriff...they want you GONE by around 60. It's written into their contracts
Reason probably being is due to the mental and physical fitness required to do the day to day activities of the job.
Why Congress and the President isn't held in the same regard is crazy.