Children who are smart enough to get access to a given vice without getting caught are more likely to be mature enough to be able to cope with that vice.
To get reliable access you either need to convince an adult to give you access (which is always game over) or you need to engage in some kind of future planning, which is a similar skill set as the one necessary to notice that getting addicted to cancer thing might be a bad idea. Stealing uncle Roy's marlboro doesn't work because uncle Roy is generally then going to notice that they're going missing and either start securing them better or deduce where they're going and visit some punishment on the kid.
If Roy doesn't care then you have a kid with an adult who gives them access, which is the scenario where none of this is going to work. Even if you required government IDs with hourly retina scans, it doesn't work if Roy is willing to let the kids hold the device up to his face whenever they want.
I only disagree with the just-so notion that kids who have an Uncle Roy are somehow better able to cope with the consequences. Ability to access something is (IMHO) pretty uncorrelated with the ability to cope with the consequences.
The original claim wasn't that the kids with an Uncle Roy would be better able to cope, it's that the kids are who can devise another way to get past even if they didn't. Then the latter kids make up a larger proportion of the ones who can get past because they have two paths to do it instead of one. And the former ones are the ones we can't reach regardless.
Kids with low parental supervision who steal uncle Roy's marlboro are more likely to be able to cope with tobacco addiction?
Do you have any reasons to think this might be the case? Studies, research, a well thought-out article?