I started at 3 or 4 with an air rifle in the back yard. By 5 or 6 I was shooting my dad's 10/22 and any gun that I could hold in my hands.
It really depends on the kid, and to a larger extent, the adult teaching them. If the kid is big enough to hold a gun, and can handle the recoil both physically and mentally, then by all means start them out strong!
You have the right idea using a .22lr to start. Just make sure he knows and can respect the rules before he starts shooting. Also, you need to maintain 100% control of everything when you are starting out a young kid. Get a bench rest of some sort, hold up the gun for him and basically let him do nothing but pull the trigger the first few times. Then you can progress to letting him aim, load, and eventually handle the gun all on his own.
But yes, 6 is plenty old enough physically, but you are the only one that can decide if he's ready mentally (following rules and directions.)
"The tree of liberty must be refreshed from time to time, with the blood of patriots and tyrants. It is its natural manure."
Check me out on YouTube!
Shooting, Engineering, and Dogs. What's not to love?