Well, you can get an airsoft gun anywhere, but you have to be 18 to make the purchase and play airsoft, unless you have permission from your parents. Like I'm fourteen but my parents let me collect airsoft guns, I just can buy them by myself.
old enough to hold it up.
Yes
It depends on the country where you live so say 16 years others say 18 years old.
You can't. You will have to sell your old air gun, then use the money towards a new PCP air rifle.
Mis-use
Yes.
You will need to specify the use.
You don't need one. The darts which are made to use in air rifles are designed to be used in a standard, unmodified, breech loading air rifle. Just be sure the darts you use are the appropriate calibre.
yes it will not exsplode
The rifleman as seen on the old TV series used a Winchester model 1892 rifle,or a reproduction of that rifle.
Not usually. There are several factors involved besides the spring, but the .22 will usually use a stronger spring to propel the heavier pellet.
There are air rifles and there are air guns. These are guns that use air or C02 to fire a BB or Pellet, not gunpowder. A DAISY RED RYDER BB Rifle would be considered an "air rifle." There are several companies that make air guns and air rifles. Gamo, Daisy, Crosman, and Umarex are a few of these companies.