BDS writes:

How about driving while impaired by age? It's the same as any other
impairment, isn't it?

Age doesn't produce any significant impairment until very late in life, and
even that is highly variable.

Why is that particular form of impairment acceptable?

Because it is so minor. In fact, wisdom and experience gained with age often
more than compensate for any minor impairment that might come with age, which
is why older pilots are often better overall than younger pilots (also, young,
stupid pilots tend to die faster).