Germany

i used to think it was a bad idea, because i know how much learning and growing up 16 years old still need to do. then i grew older and realized how many adults by age never did that.

Exactly. If I'm being honest I probably was a more informed (though not necessarily wiser) voter at 17 than I'm now. Of course we can argue that on average it goes the other way and being a new junkie at 17 like I was is most definitely not the norm, but if we let such statistics rule, we'd have to re-think the whole voting system. E.g. in Germany the school system provides a wonderful and easy solution to let only "Bildungsbürger" vote. Or we can argue with the fact that girls and women tend to score worse on tests regarding general knowledge (including the politics category), the male affinity for radical solutions, the fact that the human brain starts decaying it's 30s and so on. But in the end these statistics only give information about groups, not individuals. So I don't feel comfortable with using them as long as there's any significant number of people in that group who are able to make a conscious decision about their vote. That isn't the case with newborns, but it very much is the case with teenagers.

Tl;dr: Give me an attribute and I'll find you a statistic that you can use to explain why people with that attribute should be excluded from voting.