What is the gun license age?
There is no such thing as a gun license in the US. There are concealed carry permits/licenses and dealers licenses.
"License" is a legal requirement- and laws vary from state to state, country to country. In some places you must have a license to possess a gun, some places a license to carry a gun, etc. Without knowing WHERE you are (we get questions from dozens of nations) and WHAT kind of license you mean, it is impossible for us to give you a meaningful answer. Sorry.