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.
You can't just quit, and the military considers it a crime for you to leave your unit or disobey an order. A recruiter may tell you that you can give the military a try and automatically get out after six months if you don't like it. This is not true.