In most places, no.
An M16 is an Assault Rifle used by the military. I somehow doubt that a 10 year old would fit inside of one.
So long as they're properly supervised by a parent or legal guardian, have been taught proper safety fundamentals, and heed by them, I don't really see why they wouldn't be able to.
Have never heard of an underwater assault rifle. In the US, you must be 18 to buy a rifle, and 21 to buy a fully automatic rifle.
No
It depends on what type of gun. If it is a small to medium sixed gun (handgun or small rifle) 10 year old children can fire them. If you're looking to shoot shot guns, assault rifles, or a sniper rifle you may want to be over 13 or 14. If you're an adult you're fine shooting basically any gun.
An 18 year-old can buy a Title I long gun, which includes semi-automatic rifles. For a true, full auto assault rifle, which is restricted under the National Firearms Act and first requires approval for a Class III tax stamp to purchase, you must be 21, as per federal law.
18 to buy a rifle or shotgun, 21 to buy a handgun or assault rifle.
18 for a military style semi automatic. For a true assault rifle, which would be an NFA Class III full auto weapon, 21.
If you mean an weapon capable of both semi-automatic and automatic fire, you need to go the BATFE website.
Yes, they can be charged with assault. They may even be charged as an adult.
She could be charged with assault. As an adult, an assault against a minor is going to be pretty serious.