First off -- congrats on being a concerned and responsible parent! This is a very important question.
My principles on the matter are these:
1) Never, ever hide your guns or your shooting habits/hobbies/activities from your children. Always be completely open with them and answer any question and allow access any time they show interest -- no matter how young. Every time a kid wants to know something -- tell them. Every time they want to touch a gun, help them to do so (safely). Hiding things just screws up what should be a wonderful part of your relationship, and sets the stage for confusion, mistrust, and sneaking around as they get older.
2) Every opportunity to answer a question and help them to touch and explore your guns is a critical teaching moment to instill the four safety rules. Show them your own rigorous safety practices, teach them to follow the same, and GUN-proof your kids, rather than trying to KID-proof your guns.
3) Shooting can start as soon as the kid shows interest. My daughter and oldest son were both pulling triggers at age 3. I say "pulling triggers" as they were sitting in my lap, with my arms around them and my hands over theirs, controlling the firearms. But they wanted to, they were not scared (and certainly were not PUSHED into it), and making the guns go bang was always a reward for being able to recite a safety rule or two. FIND SAFETY GEAR THAT'S KID-SIZED! It is available, pretty easily these days, and must fit them well to do its job.
4) Everything is at the child's pace. No reason to rush or push them into shooting, or to make them continue at it for one second longer than their interest and enthusiasm holds out. Better to leave 'em wanting more than to tire them out, bore them, or (heaven forbid) make them frightened or hurt by the guns.