You need to contact your local authorities.
You need to consult with a barrister/lawyer
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.
10 maybe
you can be 10 but you need a hunters license
That depends if your state requires a license. Most don't.
In New Zealand you have to be 18 to own a gun licence.
Not to my knowledge.