You've got a good thing to research on, Kudos

I once also sat all days, night, weeks, months just to know much about this exam before taking it or advising anyone interested to sit for it.

Now to your Questions

Firstly there's no age limit for Education except NYSC in this world.
The reasons why You've have been seeing little ages on your research was because most Wealthy Students who have a good/standard background in there High School take this exam to study in UK or any part of the world

Cambridge A'Level as been an International Standard Exam to study in any University of your choice In UK and also any University of your choice in your country with chances of getting to 200L Direct and as well study any course of your choice.