We're on the same page but not the same paragraph.
If we're gonna be ageist and generalise I'll add my 20c worth. In your mid-twenties you are neither more or less sharp than in your mid-sixties or even seventies. Much has to do with genetic, developmental, cultural and particularly lifestyle factors. A twenty-something person who has lived in physical, social, or cultural deprivation is unlikely to be as sharp as a sixty-something person who has lived in relative affluence which would be marked by healthy nutrition, reasonable educational opportunities, contacts, stability, etc.
General knowledge is also culturally loaded.
Anyway, I won't rave on. The gist is that generalising about personal attributes like age, gender, etc, is a bit silly and not useful. They cannot be used as sole indicators of anything. We grow and develop out of a complex of factors and we need to implement real democracy, as you say, and also work towards a much fairer and just society than we have now.
What I am saying is that all of us, no matter our age, are affected by multiple, complex factors and that we shouldn't be too quick to generalise. Our body does wear down but this doesn't have to affect decision-making. Some go to their deaths in their 90s with a very lucid mind, some start getting dementia symptoms in their 40s and some a lot earlier, including childhood. We are all affected by a society which tends to promote health-reducing habits and environments for reasons of profit and then there are genetic factors in the mix as well. There's quite a bit of research at present on healthy old age.
From the moment we are born, we are one day closer to our death but that doesn't mean we must accept how we fare according to general trends. Our intelligence changes, not only declines, and we can play a conscious role in this.