Your max heart rate will drop with age, BUT it is somewhat genetically dispositioned. We are all used to the default calculation (220-our age) but there are better ones out there if you are willing to track some measurements like your resting HR (don't get out of bed, right when you wake up, measure your resting HR).

So when you increase your fitness, and get tested, you are not necessarily looking for a higher max HR as a big sign of success, but rather a higher AT (anaerobic threshold). When your AT is higher, you can work at higher levels without going anaerobic.

This is what we all want...as we can only stay anaerobic for a short period of time before maxing out and that lactic acid start to burn in our muscles. When you are beyond AT, your oxygen runs out and your body draws on reserves for fuel.

And, of course, we have all heard it before....our best fat-burning zone is when we are in our aerobic heart ranges. (65-85% of our max). Not that we worry about that right? Yeah, right.

Good reads: Sally Edwards: Heart rate Monitor Book
Ed Burke: Precision Heart Rate Training
Phil Maffetone: High Performance Heart
Stu Mittleman: Slow Burn
Each of these authors has more than 1 book out also.