I'd say about 3 people I know in the 30-40 range have children 10+, so it's becoming an issue. We all are young enough to have ditched land lines and it is starting to become an issue.
Where I live there is no technical rule, but 12 gives you the presumption of not being a terrible parent if something does happen. When my girlfriend's daughter was 11 we decided it made more sense to let her read if we went to the store for a short trip (less than an hour) than to force her to tag along. We had a gut reaction of hell no, you don't need a phone when she asked for one for Christmas, but eventually, upon thinking about it, decided that it would be convenient for her to get in touch with us (for everyone involved) when she was somewhere else (be it home, or, at a friends, or even at after-care in school trying to arrange hanging out with a friend). Once that decision was made, we went with smart phone, because she could keep in touch with her international grandparents (via social media and email of photos of things in her life), and access to real information seemed like a good thing too, essentially, the same reasons adults use smart phones seemed to apply to children too.
Even if one is to use 12 as the cut-off for home alone, that's two years before credibly being able to afford a phone on one's own, I was honestly curious what somebody completely out of my social bubble's decision was, thanks.
PS, I don't know why I didn't think of no SIM 911 as an option, and at this point, something like skype (on wifi) on a no SIM card would do the trick for the at home situation, but that was less smoothe back then.