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.