What age do you feel firearms training is appropriate?

I do not think 10 is too young, in fact I wonder why you waited till 10. When I was about 5, my dad very carefully built a range in the garage and we shot bb guns into a pellet trap, under his close supervision. I remember vaguely that I took right to it and kept most on the hits in the black on the target. About a year later, at 6, I was firing .22 rifles in the range at Ft. Wachooka. My older brother and sister were doing it all too.

On the other hand, when I took 2 of my grandsons, 5 and 7, shooting bb guns, they could not hit the target, I surmized they did not comprehend the basics of a good sight picture, which I carefully taught them, and I closed down the range.

I think my point is that a child can shoot when the child is ready and the OIC/NCIOC is ready.

The sooner they understand what a weapon does, the safer they will be with a weapon.

I believe you need to teach your children the minute they pick up and squirt gun and old enough to understand the basics. I am a firm believer if you have firearms in the residence everyone should know the basics about handling and deploying them.

I have been exposing and educating my kids about firearms since they were very young. They would see me cleaning and caring for the weapons after using them, and then I would take them to watch me shoot.
I did not have anything really suitable for them to shoot, however as each kid was 'ready' (as determined by me) I helped them shoot a .38 revolver. Over time, I started exposing them to a 5.56mm AR, which they really liked!
Now they mostly shoot a .22 pistol and .22 rifle, but my oldest (almost 13) is getting big enough to handle some of the larger calibers including most recently a .45 (with my dedicated help).

BL: Each kid has been different, however they are comfortable with my weapons, know they are not to handle them without supervision (they are secured as well) and know how to handle when it is time to do so. We started shooting when they were between 7-10 with hands on assistance, moving to close supervision. I look forward to being able to have them shoot almost autonomously!
They love going to the range and are getting to be pretty good shots!

Define "training".

I would say you start training them once they understand what a firearm is. They may not be firing it or even handling it but teaching them about firearms and proper safety is still training. They are never to young to start picking up on how you handle firearms.

I grew up with firearms all around me. My father taught me to shoot before I started grade school. I was never scared of firearms but I knew how to safely use one and what they were capable of.