Latitude's advice, free or not, is good. The general rule of 220 minus age, like all generalisations, tells you nothing about your particular circumstances. I'm 55 going on 56 and have touched 186 in the last year. My resting HR is around 52, and I regularly go over 160 without feeling I'm working too hard. On a climb I'll start to back off at 170 because I know from experience that I can sustain that level for only a few minutes and then I'm blown.

I have friends who find it difficult to get their HR above 150, and others who average over 160 on a fast ride. If you're worried about it, talk to a doctor as others have suggested. But generally speaking, I'd suggest that you proceed on the basis of what feels OK, as opposed to some hypothetical number.