I started my two girls when they were 4 and 6. At first the four year old just came along to watch and occasionally, pull the trigger while daddy held the .22 rifle....
But about 5 1/2 years old, she started doing it all by herself.
They will be 7 & 9 this July and August, and both handle .22 rifle's just fine all by themselves...
The 9 year old loves the MP5, but it is still too heavy for the 7 year old.
Start em EARLY... you can always have them just watch until they feel they are ready to do it themselves...
Oh, and get reactive targets... my girls don't enjoy paper targets... but they love watermelons, shook up soda cans, clay pigeons, army men, etc....
Semper Fidelis,
Kent