Need a little more information. In most U.S. states, you have to be 21 or older to obtain a handgun carry permit, if that's what you meant.
You need to tell us more about the "gun license" and for what area. There are many different gun licenses.
There is no such thing as a gun license in the US. There are concealed carry permits/licenses and dealers licenses.
Gun Dealers and gunsmiths
It depends on where you are.
Texas.
yes
You do not need licenses for airsoft guns, as they are not firearms.
In the US, the BATFE issues licenses for gun dealers.
Depends on which license, and where you are. You will have to be specific.
There are two main types of gun licenses in the United States: concealed carry permits (which allow individuals to carry a concealed weapon) and firearms licenses (which regulate ownership and purchase of firearms). These licenses vary by state in terms of requirements and restrictions.
The agency that issues the license will tell you.
18 for rifles, 21 for handguns...