I disagree. Most of the evidence suggests that intense "extreme" exercise is beneficial, even for the very elderly. It's a mistake to think that going hard will lead to injury or death. Actually, failing to go hard probably has more negative consequences, and the notion that the over fifties shouldn't exercise vigorously probably appeals more because we are lazy and prefer to be comfortable than because it has any intrinsic merit.

Of course, drving oneself to complete exhaustion by riding at threshold for hours at a time may not be the smartest idea. But going as hard as possible for short periods seems to be extremely beneficial.