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.
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.
Medal of Honor European Assault is rated teen so you can buy it.
An M16 is an Assault Rifle used by the military. I somehow doubt that a 10 year old would fit inside of one.
No
For a transferrable, select fire, NFA tax stamped rifle, 21. For a semi auto military styled rifle, 18. This law pertains to the US, and laws will vary in other countries.
In MOST states it is 18 to own any shoulder-fired weapon. In order to own what the military considers an assault rifle, you also have to have a Class 3 Federal Firearms Lisence, but some states and cities definition of an 'assault rifle' includes anything that looks dangerous and NO ONE is allowed to own one there.
A 12 year old can't "own" a gun, since you have to be 18 to buy a rifle and 21 to buy a handgun.
Depends on Texas law.
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.