In the UK, the sale is already "prevented" in that's it's illegal to sell games to consumers outside of their rated ages. Done. That's as much as you could ever actually do. That's been in places for years,
I thought it was obvious I already knew that...?
That piece of legislation is fine but if it's never enforced (like in the UK)
Huh?
And even with the law? Still you don't *stop* anything. Nothing. Kids of parents that don't keep an eye, don't run a household with consequences, etc. still smoke, drink, have sex, play violent videogames, bully, steal, watch porn and whatever else.
More black and white viewpoints from you, that all kids are either angels or devils. There are plenty of children who will attempt to do many of those things and then just give up if they fail, meaning the existence of the law had an effect.
The rest of your post I'm not going to even individually comment on as it shows you pretty much ignored half of mine. For the parents to be able to take action over children having these items requires the parents to know their children have them, and I already explained that for the parents to know about them oversteps the boundaries of freedom that British teenage children generally have.
One more time: over here it is normal for children to go out on their own with friends at a young age, unsupervised. Thus it is only appropriate the duty of care for some things like sale of unsuitable goods temporarily shifts from the distant parents to the adults around them.