I'm reading your post and it reads just the same as what applies to many adults.
I know that I would have voted for a liar with a corrupt past, because of facebook ads of their party I assume. “oh look, they are apologizing and they regret it! they look so honest!”
I can't even get started how many politicians have a corrupt past here in Germany and got plenty of votes.
nowadays? they just post a tiktok video that they’ll give money to all below 20 if they are elected
Here in Germany parties actively ran on the promise of raising and fixing the pension levels in an already unsustainable system. Alongside other gifts to certain voter bases. The one left out (I assume partially because they are not able to vote): The youth.
I also think you vastly overestimate the amount of influence underage voters would yield. Especially in our demographic structures and based on the fact that a significantly lower share of them would actually make use of it. They certainly wouldn't have the power to introduce sweeping changes against the better judgement of other voting blocks. But you are right that they might influence smaller changes.
To take one of your examples i could see that for something like the smartphone ban. But would that be so bad? It might be a good thing, but i don't think this is conclusively proven. In return it is probably something being pushed by a large majority that might not even use a smartphone on a daily basis or at the least is very far removed from the current level of technology. And it also wouldn't all need to be negative. Take for example the stop killing games petition that is quite popular on this site. That one might suddenly gain some more supporters, which are actually affected by it.
However i'd also see a need for more studies. And i probably wouldn't just make a major shift like that instantaneously, but rather in a gradual way and maybe lead with changes to smaller more local elections first. Which might give opportunities for such studies.
Yes, i think we should definitely pay more consideration to how our democratic system works on a more mechanical level, and not just specific opinions. Glad to hear i am not alone in this and i imagine that other suggestions like e.g. the use of ranked choice voting would be much less controversial than this one.
I'd say the age filter is perfect. But it only filters for the one thing it measures: age.
My argument is that (here in Germany) when i go to vote there are 4 requirements asked of me:
Citizenship (although in some more local elections i think this isn't even a requirement as e.g. other EU residents are for example also allowed to vote). Which is a binary classifier, one either has it or does not. I've had it since birth
That i am currently not stripped of my voting rights. Something that (rightfully) is done extremely rarely and on an individual basis, e.g. for high treason or bribing officials. Here in Germany it's also always a temporary measure for a maximum of 5 years.
There are some limitations based on residence. For example federal elections seem to require that you've lived at least 3 months in Germany during the past 25 years (with exceptions for some professions).
Age, currently being over 18 in federal elections, 16 in some state and regional ones. Again a binary classifier, once you pass the threshold it becomes irrelevant.
The last aspect of course is that it is done so by ones own free will.
Now this i think is what you are going for, but i don't think it has anything to do with the age requirement. It's required from anyone that votes regardless of age. And in fact we already have a system in place that we deem sufficient enough to decide it, since we already have citizens where it might be in question like e.g. someone with an intelectual disability which can voice their wish to vote and sometimes receive help in doing so. Similarly if you have physical issues and are e.g. blind or can't read you can get support to allow you to vote. Prisoners who are not able to control a lot of their circumstances are able to vote. Notably we do not care about whether or not you vote "badly", for the wrong reasons, or for someone we disagree with.
The filter for this imo would be the same as for anyone else. A declaration that you want to vote and that you do so free of duress. This filter could imo be fulfilled by a child stating their wish to vote just the same. However as stated somewhere in another comment above i'd be fine with having an additional requirement here that the first vote would need to be either in person or that one would need to actively apply for it (and if not the automatic registration comes at a certain age), in which case we'd probably need to give children some options on where to do this, e.g. in school.