VO2 max is the maximum amount of oxygen that your body can take in, deliver, and use in one minute of time. Those who are fit have higher VO2 max values and can exercise more intensely than those who are not as well conditioned. It is limited both by the amount of oxygenated blood the lungs and circulatory system can process, and by the amount of oxygen the muscles can extract from the blood. It is estimated that VO2 max goes down about 1% per year.... as we get older our VO2 max decreases.

Here is a general listing of what different groups of people's VO2 max's are. They are calculated in ml/kg/min.

General Population, Female, Aged 20-29: 35-43
General Population, Male, Aged 20-29: 44-51
Highest Recorded Female (Cross-Country Skier): 74
Highest Recorded Male (Cross-Country Skier): 94

The VO2 max test reveals the capabilities of your aerobic engine. It can measure both current fitness and your athletic potential that you can improve through training. The VO2 test involves riding a stationary bike or jogging on a treadmill to exhaustion while breathing through a tube. A machine then calculates the amount of oxygen consumed at peak physical capacity and how much CO2 is produced. The more oxygen the subject's body uses while huffing and puffing, the greater the VO2 max will be.