In MOST of the US, there is no license to OWN a firearm- there are licenses to carry a concealed firearm, and licenses to hunt. While rules vary state to state, in most cases, you must be 21 to obtain a concealed weapons license, and hunters may be licensed at around 14-15 years old. You may purchase a rifle or shotgun at 18, and 21 for handguns.
You need to tell us more about the "gun license" and for what area. There are many different gun licenses.
Whether or not you can have a gun license at 20 years old depends on which US state you live in and the type of gun as the laws vary somewhat.
21
No.
There is no such thing as a gun license in the US. There are concealed carry permits/licenses and dealers licenses.
You need to define the term gun license. In most of the US there is no license to own a gun. Some states require a license to carry a gun. others to carry a gun concealed. There is also a license to make or sell guns. All are different and have different procedures.
That depends on if your state requires a license or not. Most do not.
Old enough to spell license.
Depends on where you are in the US. In most of the US, no license is required.
It depends on where you are and what you mean by gun license. If you are in the U.S. and you mean a permit to carry a gun, check the related link for information.
In the US, the BATFE issues licenses for gun dealers.
In US - no. In UK - yes. In Australia - yes.