The voting age is entirely arbitrary, and does take into account how knowledgeable or clever a person is. If it did, half the nation would be disqualified from voting.
And it is not about maturity. Age is not a guarantee of wisdom. Some people are just stupid, and the years between their 18th Birthday and their 70th Birthday will not alter that.
Some of the arguments against lowering the age on this thread are extremely disingenuous. As if most 16 year olds are not fully formed humans yet.
We are debating this now because one side believes a change (or not) will be an advantage to one side or other. So the debate unfortunately goes down those lines. Conservatives are against it because they fear 16 year olds will favour more left wing views. And lefties are in favour of it because that is probably true. But it is not really the basis for an informed argument. We all know that pensioners tend to be more conservative voting, but that is no basis for saying the voting age should be capped at 60, and anyone above that age disenfranchised.
Probably best though, to leave it as it is.