Since the 1970s, the general suggestion for determining your maximum target health rate has been to subtract your age from 220. However, as a New York Times article points out, that formula is not particularly accurate.
The traditional formula underestimates your heart rate regardless of your age, but it starts getting inaccurate if you're older than 30. Unfortunately, the more accurate formula is a lot harder to remember:
Instead, according to his group's recent examinations of 3,320 men and women between the ages of 19 and 89 who participated in a wide-ranging health study in Norway, a better formulation for both sexes would be 211 minus 0.64 per cent of age. Should your brain, like mine, revolt at the thought of the maths required to determine maximum heart rate with those numbers, Dr. Wisloff's lab has posted a simple calculator online.
Of course, exercising in any way is helpful, and you don't always need to know your maximum heart rate (it is helpful for certain training like the seven-minute workout though), but it's often helpful to see how hard you're really pushing yourself.
Ask Well: Maximal Heart Rate [New York Times]