Quoted:
A while back there was a running discussion on this board about training with/by HR that included some discussion on training levels/zones.  Specifically there was disagreement about whether one needed to perform some type of testing to establish training intensity guidelines or if a formula like 220-age was more appropriate.   The linked article below was published in the Journal of Exercise PhysiologyOnline:  

The Surprising History of the "HRmax=220-age" Equation

Some of the conclusions:

-Research since 1971 has revealed the error in HRmax estimation, and there remains no formula that provides acceptable accuracy of HRmax prediction.
-If HRmax needs to be estimated, then population specific formulae should be used. However, the most accurate general equation is that of Inbar (17) (Table 3); HRmax=205.8-0.685(age). Nevertheless, the error(Sxy=6.4 b/min) is still unacceptably large.
-The use of HRmax is most prevalent in the fitness industry, and the people who work in these facilities mainly have a terminal undergraduate degree in exercise science or related fields. These students/graduates need to be better educated in statistics to recognize and understand the concept of prediction error, and the practical consequences of relying on an equation with a large standard error of estimate (Sxy).