My feeling on black belts is this: One test to rule them all.
In other words, the test is the same regardless of who takes it or how old they are. The test should be geared to and challenging for the average adult student (16+). It should be a rigourous test. If a twelve year old can pass that test, then kudos, give him his belt. If an eighty three year old great grandmother can pass it, give her her belt.
That way, there is no ageism issues. The instructor should then simply inform the student that they are ready or not ready to test. That avoids kids or elderly students who are not able to withstand blows from adult students getting seriously injured during the sparring segment of the test.
In my view, a black belt is a sign that the student has learned, is proficient in, and can make practical use of the system, whatever it is. Realistically, there are physical requirements to being able to do that. Being youthful or elderly certainly can impact one's physical ability to execute the techniques proficiently.
A black belt should be able to practically use the system against anyone, regardless of age. Not beat anyone, but make practical use of the system (blocks, kicks, punches, footwork, etc.).
Daniel