Age discrimination.
The problem is not that some of them get old. The problem is how to get rid of the ones that either do not or can not perform regardless of age.
All federal jobs need to pass Congress’s constitutional Article I, Section 8-limited power test.
More specifically, if Congress cannot justify a given federal job under it’s Section 8-limited powers then Congress needs to do the following.
Basically, Congress needs to stop appropriating unconstitutional taxes for constitutionally indefensible federal jobs so that the states can find new revenues to hire such employees away from the feds.
"Congress is not empowered to tax for those purposes which are within the exclusive province of the States."—Justice John Marshall, Gibbons v. Ogden, 1824.
Conceptually, the former federal employee will then do the same work for a state that they were doing for the feds.
”... the care of the property, the liberty, and the life of the citizen, under the solemn sanction of an oath imposed by your Federal Constitution, is in the States, and not in the Federal Government [emphases added].” —Rep. John Bingham, Congressional Globe, 1866. (See about middle of 3rd column.)
"It is one of the happy incidents of the federal system that a single courageous State may, if its citizens choose, serve as a laboratory; and try novel social and economic experiments without risk to the rest of the country.” — Justice Brandeis, Laboratories of democracy.
Note that the constitutional limit on states as laboratories of democracy is that states cannot abridge constitutionally enumerated rights.
”From the accepted doctrine that the United States is a government of delegated powers, it follows that those not expressly granted, or reasonably to be implied from such as are conferred, are reserved to the states, or to the people. To forestall any suggestion to the contrary, the Tenth Amendment was adopted. The same proposition, otherwise stated, is that powers not granted are prohibited [emphasis added].” —United States v. Butler, 1936.