Your permit to do what, and where?
You will have to define "gun permit". It means different things in different places. In some areas, you must have a permit to possess a gun, in some places a permit to carry a gun, and in others, a permit to carry a gun concealed. depends on the laws where you live- and we get questions from all 50 US States and multiple nations other than the US.
Yes and no, in some places in the state you can have them, others you cannot. Then in some places you need a permit, and who will use it must be written on it. Then in some places, they aren't illegal, but they wont be put up because of how "dangerous" they are.
lots of places if you have a permit
Yes, in most places, you can get a permit at 18 years old.
Yes, in most places, you can get a permit at 18 years old.
The military may cover it, they may not. Some places do not permit it. Whoever would have to check with their person in charge.
Yes, in most places, you need a driver's permit at 18 to legally drive.
To the best of my knowledge, no place exists in the USA where you can start a fleamarket without a permit. Generally a business permit is required, and a sales tax license. Some places may require a special permit for flea markets. This is a vague answer, but it can be improved upon by others.
If you mean a license to sell guns, it is possible, IF your zoning laws permit operating a business at home. Some places do, some do't.
Not normally, 14 is typically the minimum to get a work permit, but it could be higher in some places.
It has to be peaceful, no blocking of public places, has to be on one place, need a permit and security provisions in some cases.
No. Child labor laws prevent it. Some places will hire at 16 with a work permit.