As a grad student in Exercise & Sport Science, I can assure you that what everyone's said here is true about the inaccuracy of 220-age. That's really only a prediction- if you took everyone in the world and put them through a maximal exercise test, you'd most often find that they're within 5 or so BPM of 220-age.

When you start dealing with people who aren't "normal" is when that formula gets thrown out the window. Athletes and others who choose to maintain a higher fitness level than their sedentary counterparts will not experience the same decline in maximum heart rate as other people- it IS going to go down as you get older, but the rate of decline just isn't as fast.

The best way to measure what your max heart rate really is is to go to an exercise testing lab. Typically, in our human performance lab at University of Memphis, we'll put someone on a treadmill and perform a maximal exercise test. During the test, we measure VO2max, lactate threshold, and use an EKG to monitor heartrate. You can have a maximal test performed at a doctor's office or cardiac clinic, but they'll usually only gather EKG information in that type of setting (VO2max and lactate threshold are more along the lines of performance-related data rather than health related).

This information is most useful to people who want to form a detailed training program and/or want to monitor how their fitness changes in resonse to a training program.