There might be a date of manufacture on it. If there isn't, someone knowledgeable about guns might be able to tell you when it was manufactured.
New Answer-------------------
There are a few ways to get an answer to the date of your air gun. You can ask here in "Answers. com." Give the model number. Name on the air gun and the serial number. State who made the air gun. ie Daisy, Gamo, Crosman Benjamin or whoever. This is a place to start.
Make sure your question is placed in the "Air gun and Air rifle" category so I can find it.
Or
Locate a "bluebook of airguns" at a book store. There you will find almost all the answers to your question.
If your air gun is a Crosman, then Crosman has a web page to date your air gun. In fact you can write to most manufactures and ask them directly. most all of them have a web page with a contact link.
the proper size
That decision is up to the parents. Is the child mature enough to use the gun with care. Do they know how to handle the gun safely. Do they know it's NOT a Toy. All these things should be considered.
In most states you must be 18 years old
Colt makes a 1911-A1 Pellet gun made by Umarex. I know of no BB gun unless you are asking about an Air Soft gun. The Pellet gun is very good. I can not answer about an Air soft gun.
In most states you must be 18 years old
In most states, you just have to be 18 years old.
It depends on the model and the caliber of the Pellet gun.
If you are talking about Air Soft guns, 18 years old to buy one.
I think you mean pellet gun. That is a gun that fires a small lead pellet but does not use gun powder. It uses compressed air or compressed gasses to push the pellet.
It all depends on the model and who made the pellet gun. I suggest that you re-post the queston with the model and make of the pellet gun to get an answer.
Any pellet gun should be handled like a real firearm. Yes you can be killed with a pellet gun under the right circumstances.
A pellet gun is not adequate for protection, and will get you arrested.