16, 17 years is fine, generally youth's this age are attentive enough to benefit from training. I was training as a infantryman in the Army at 17.
And as JF says.. get em early start them off right and you will have fewer problems going forward when they join the wider community.
The proliferation of inexpensive guns, and growing popularity of Airsoft is driving the demographic younger.. these guys are going to play one way or another.. Better they get set up right.
CAPS Range Officer
Toronto Downtown Age Verifier
If the tongue could cut as the sword does, the dead would be infinite