Guns with a power above 0.8 joules (which includes everything above a very gentle pellet gun) may not be sold to under-18s. However, from the age of 16 it is legal to possess a 7th-category weapon...
It depends on the state and county that you live in. Usually you have to be 18 to purchase an air rifle. Some states require that you have a gun license to own one; this is common in New York. Other...