I posted this in the 50+ subforum, but it's worth reposting here. If you're one of those people for whom the 220-age formula underestimates your MaxHR (like me), this helps demonstrate how you won't be pushing yourself hard enough to evoke the best training response. That is, you'll be cheating yourself:
This is from a ramp test I did where the target wattage increases every minute. You go till failure, so it is truly a max effort. You can see how the red line (HR) and the magenta line (power) ramp up. I drew a line where my HR crossed the 220-age predicted maximum. If I had quit there, and set my workout zones accordingly, I'd have left a whole lotta watts on the table.