This is really great but it is also slightly troubling. While there are a number of popular calculations or formulae around, arguably all giving a similar answer, it is worth looking into. It is, at this point, worthwhile for me to remind you I'm not a Doctor or qualified in anyway to give health or sports training advice to anyone out there. All I'm doing is commenting on these things from my own perspective. Okay, that's got that out of the way.
The popular formula is this:
220 - Age = Maximum Heart Beat / minute
For me this works out to be 170 and I know that the recommended rate for improved aerobic exercise is about 80% of the maximum heart rate - so my ideal maximum should be around 136bpm . When I run at a steady 7 to 7.5mph it feels quite comfortable and I can have a coherent conversation and yet my heart rate is around 170bpm and will rise up to 185bpm if I run as fast as I can. In theory puts me into a danger zone but it feels fine to me. I think I'm in good shape; I don't smoke, drink alcohol or eat much cheese and my resting heart beat is about 50bpm.
A quick trawl around Google shows an excellent article by Dr Alison McConnell, sports and exercise physiologist,