Also, 12 year olds are less mature than 18 year olds. It may be that the level of immaturity in voters you'll" get from adding people ages 12-17 is just too large to be acceptable.

"Maturity" isn't obviously a desirable thing. What people tend to describe as 'maturity' seems to be a developed ability to signal conformity and if anything is negative causal influence on the application of reasoned judgement. People learn that it is 'mature' to not ask (or even think to ask) questions about why the cherished beliefs are obviously self-contradicting nonsense, for example.

I do not expect a country that allows 12-17 year olds to vote to have worse outcomes than a country that does not. Particularly given that it would almost certainly result in more voting-relevant education being given to children and so slightly less ignorance even among adults.