How do you measure VO2 Max?

VO2 max measures the size of your engine...your potential.  What is important is your HR at VO2max and your LT.  MaxHR isn't really a good guide for training.  Remember that LT is something that is specific to you and can't be compared to anyone else.  Having a higher LT than someone else doesn't mean much about how you compare to them athletically.  VO2max is your "bragging number" if you're into that kind of thing but even that doesn't necessarily dictate the better (faster) athlete.  Economy plays a huge role in who crosses the finish line first.  A big potential (VO2max) is great but if your are mechanically inefficient then you could very well be wasting a lot of that potential.

While it's generally true that your VO2max will decrease with age it isn't necessarily because of a loss in fitness.  Think of your VO2max as a measure of your potential.  It isn't a good indicator of your actual fitness level although it can be affected to some degree by training.  The more telling parameter to track as a measure of your current fitness is how your LT relates to your VO2max...or how much of that potential (VO2max) you can actually make use of.  Untrained individuals usually reach the LT at a HR about 60% of VO2max.  With training, LT can increase from 60% to above 70% or even higher.  Elite endurance athletes and top masters athletes typically have LTs at or above 80% of VO2max.  Values approaching 90% have been reported.  The lactate threshold is both responsive to training and influenced by genetics.

http://home.hia.no/~stephens/vo2max.htm

http://home.hia.no/~stephens/lacthres.htm