Triathletes are a odd group to base this on. In any case, the exact age isn't important and you would of course expect this to vary dramatically. The point was

Triathletes are a odd group to base this on. In any case, the exact age isn't important and you would of course expect this to vary dramatically. The point was that it is very far from being as simple as "performance decreases as you age". But you are correct to point out that your parent is very far off the mark. Max HR is not a useful predictor of performance, and you can not calculate it with a formula. What you want is a test to exhaustion to determine it, but these are highly dubious due to the psychological aspect, and at the end of it you might have a correct value for max HR, only to realize that it is pretty useless for any prediction purpose. Case in point, Chris Froome, winner of the Tour de France in 2015, has a max HR of around 170, yet he can sustain 450W for a hour. Meanwhile plenty of people with a max HR of 200 and beyond can't climb stairs without heavy breathing.