I'm 58. I cruise at 137-145 bpm and reaching 155 takes but say 10-15 seconds approaching max effort. 165-170 takes 20 seconds more up a hill at max effort or a bit longer on the flats on high gear. The caveat. I used to ride 6 days a week and killed it each ride (an hour to 90 minutes). But I realized that I was not recovering so I switched to 3-4 days a week with a day of rest between rides and moderate ride pace alternate days. That day or two days of rest is everything. I also limit interval training to one day a week. My doc (former NASA doc) says to watch the high heart rate (165-170+) peak to it but not stay or hang there for long (a few seconds) there. My max is supposed to be 162 Karvonen). So that's caused me to understand how to peak on a hill and not charge it too early. Illness messes things up and I'm really attuned to that though I rarely get colds/flu. I have to watch hydration like a hawk. Dehydration can mess with me, heart rate and blood pressure, and more than once I've found myself not drinking enough. I strive for a quart an hour in our area (55 F and 80%+ humidity). All my marathon fotos of the Seattle-Portland have me glistening like an 18 year old cherub and a quart an hour was barely enough. I ride with two 28 oz bottles with a 30% fruit juice 70% water mix, pinch of salt.