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 need to be 18. You can apply to join when you are 17 years and 9 months old, so that you're ready to join on your 18th birthday. To join as a soldier, you must apply by the time you are 49 years and 11 months.