My experience in the US, for something that's not a BB gun, I'd say 10 or 11, but that's not a hard and fast limit. It's usually a small caliber (like .22) or something. Hunting used to start at 12 way back, although these days I've heard some places allow hunting as early as 8..? The truth is it depends on the individual, and how well they can be trained. This video gives a pretty good overview of "milestones" someone should reach before owning a gun: https://www.youtube.com/watch?v=5fRJTyU851Y
