Here in Germany it's fairly common for kids aged perhaps six or seven and up to take public transport by themselves. They might have a dumb phone or occasionally a smart watch, but I rarely see them with their own smart phones.

One of the most important principles of a public transport system should be that it's accessible to all in a lowest-common-denominator sort of way. Anything beyond that is also good to have, but if you don't have that basic level of accessibility, then it's not really a public transport system, it's a luxury transport system. And there are already plenty of luxury transport systems around.

Also, my last phone died on me fairly often, I don't think it's nearly as unusual as event as you're making it out to be.