I went to high school with a kid that died at 17 in a car accident. I'm 37 and my heart is still beating. Not trying to be pedantic here, I've heard that same thing, the problem is general rules tend to get caught up on the specifics. The devil is always in the details.

Probably more to the point, exercise raises your heart rate temporarily, but it makes your heart stronger and that means it doesn't have to beat as often to do the same job. That's why people lower their resting heart rate through regular exercise. Somebody else did the math in this thread, you might elevate your HR by 70 beats for an hour, and lower it by 10 for the other 23 hours in the day; in the end you come out ahead. And even if the thing about having a set (or max?) number of beats is flat out wrong, you still benefit from having a strong heart.