If you want to have universal suffrage, you can't rule out elderly people or rule people out on the basis of their IQ. You can, however, set at the age that people legally become adults (as it is now), on the basis that, at this point, they should be mature enough to use their vote, or understand what it means not to use it. The voting age was lowered from 21 to 18 in 1969 because that was in line with what people, at that time, considered to be the age of adulthood. Research showing the differences between adult and teenage brains and the fact that the brain continues developing for much longer than thought didn't yield those findings until much more recently. Funnily enough, those results suggest that 21 was probably more accurate than 18 as the age of majority.
Given the increasing incidences of stress and anxiety being recorded in teenagers, I would have thought that it was better to let them mature for longer before forcing the weights of the adult world upon them; giving them time to learn resilience and about themselves. This idea that making them become adults earlier is progressive seems nonsensical to me. It would be better to revise age limits for things like voting, joining the army up rather than down, given what we now know about development of the brain. Certainly, that's the trend within education, with them having to remain with some kind formal setting until 18.
