This happens because the temperature at which oxygen changes from a gas to a liquid is about 13 degrees Celsius above that of nitrogen. The oxygen can be seen dripping from the can just as on a humid day you can see drops of water condense and drip from a can you take from the refrigerator.
Though you technically can join the military up to age 42, if you're 35 or over you'll need an age waiver if you've never been in the military before.
To join the Army Reserve, you must be a U.S. citizen or a resident alien between the ages of 17 and 27. Seventeen-year olds need parental consent. You must have a high school diploma, and have no more than two dependents.