Citation needed
AFAIK it's not remotely true that "we all lose our mental faculties". Plenty of people do not
But a saving grace is the degradation can be combated with exercise both mental/physical.
I am not sure how much of aging is understood, so I hesitate to mention stuff like DNA damage.
One thing's for sure though: we're still in the stone age of neurology.
Now of course we are aware that the brain has plasticity. It can rewire itself, grow, learn.
Re: citation needed : failed literature search link needed as pre-requisite