Those in this thread with diagnosed arrythmias or coronary artery disease have reason to be careful in one way or another. The rest of us should not worry about going too hard, because we're probably not going hard enough.
I can't be bothered to look up and link the references, but you can google for them. There is decent evidence that brief periods of intense activity produces better health benefits for the elderly in terms of various metabolic markers, VO2 max etc than does extended moderate activity. And by intense activity I mean maxing out, going as hard as you can, for a minute or two at a time. And the evidence relating to increased incidence of arrythmias among endurance athletes appears to be related not to maximal efforts, but to going hardish - around threshold - for very long periods: we're talking ironman, pro cyclists, multiple marathons, that sort of thing.
It's likely that the best thing you can do for your heart health is to go steady most of the time but make sure you go very hard indeed for a few minutes a couple of times a week. A nice couple of hours ride at recovery pace with a sprint up a decent hill in the middle of it is probably ideal. For the record, I'm 65 and regularly see an HR in the 170s.