you have to be 12 but you have to do a safety test first
In my state you must be able to read and write English to get a 'junior" license.
5
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.
That depends on if your state requires a license or not. Most do not.
Old enough to spell license.
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.
12 for your junior licence and 16 full your full
10 maybe
you can be 10 but you need a hunters license