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 cannot enlist in that branch at 17, even with parental consent. Each of the military service academies share the same age requirements for students wishing to enroll: Each student must be at least 17 years old and no older than 22 years.