My family bred gun dogs and my cousins and I all held shotgun licences from about the age of 13, although a friend of mine had one at about 11, I think (farming family).
I don't really know the law these days, but I think it would be quite difficult to use a shotgun below that age - they're quite unwieldy and you have to be reasonably strong to manage the kickback.
You do have to jump through multiple hoops to get a licence, and we were never, EVER allowed to handle or be around guns unsupervised - my uncle was the keyholder to the gun cabinet and to this day, I have no idea where he kept the keys (no guns anymore, we're all townies now!).
We were also taught incredibly stringent gun safety from before we were even allowed to touch a gun, and if there was the slightest hint that you weren't keeping your nose clean, all privileges were revoked instantly - we knew from a very young age that these were potentially lethal weapons and to be handled with the utmost respect.