Personal experience says It really depends on the kid. I know a 7 year old who can shoot safely with minimum supervision. I also know adults who I wouldn’t let park my car.
I started with gun safety with my two year old as soon as he showed any interest. It’s never too early for that. We refresh on the training often. I was shooting a single shot .22 when I was pretty young. Probably 4 or 5. I had so many safety talks as a toddler I could recite the rules easier than my ABC’s though.

I prefer a single shot .22 for kids. It gives you a lot of control and minimizes the chance for accidents. Any of the old rolling block, like a Stevens, or modern bolt action single shot youth .22 rifles would be my recommendation. Both of my sons have savage rascals. Crickets are okay too.
Some people start their kids in a semi auto like a 10/22 or the S&W M&P 15. It wouldn’t be my recommendation, but I know people who have gone that route.