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.