Got my son started just messing around with a BB gun at about 3 1/2 but just for fun. This yr on his 5th birthday I got him a cricket 22 that's fits him great and I like the fact it's a bolt action so he can learn the action early on. He has shot my 10/22 as well but it's hard for most kids to shoulder the stock and still reach the trigger the cricket is shorter and a better fit. We have played around with the cricket a few times but yesterday I took him to shoot and wanted to focus on the basics especially sight picture with open sights and trigger pull. His first couple targets weren't too great but he slowly was understanding that in order to hit the target you have to keep the same sight picture. We only shot at 25 yds but he did pretty well on his last 10 shots at a clean target. He loaded, aimed and shot all 10 rds on his own I was just standing there to ensure he was safe.
I would say age is always going to be based on their interest in shooting, their attention span, and their patience. Nothing wrong with giving it a shot at any age, maybe they are interested and pick it up quick maybe they need some more time.
His last target of the day