The fact that scooters can be used in such ways is not a compelling case that they're more dangerous than cars. If anything, what strikes me is that a scooter on the sidewalk seems outrageous to some people, by contrast, largely because it's just a banal fact we're all accustomed to that cars kill people by the tens of thousands every year.

The exact same can be said for bicycles. They go the same speed, they have to be used in the street and not the sidewalk, and bicyclists can be just as careless and oblivious on the road. Yet there is no age restriction for riding a bike on a roadway; you can bike down a 45mph road perfectly legally the day you get your training wheels removed.

The problem is not the scooters and who rides them, it's the danger posed by cars. Cars have to be physically separated from other forms of transport. Buses should get their own lane to beat car traffic, bikes and other lower speed vehicles like scooters or even runners should stay in the bike lane. This is a transportation issue, not a scooter issue, and you can't fix it with an age restriction.