Source: "endurance sports also peaks around 27 years of age" I have seen most triathletes hitting their best time in their early 30s. Chrissie Wellington actually only started professional ironman in her 30s. She was ironman champion in 2007(30),2008(31),2009(32),2011(34).

Also cardiac output is more correctly a function of stroke volume, how much blood is pumped for each stroke. Therefore very well trained athletes actually have a lower max hr.

Also cardiovascular drift can increase your max HR, this happens when it's very hot or you have lost a lot of water due to sweating, this causes stroke volume to decrease and heart rate to increase while you still at the same physical output. You hear ppl saying my heart rate was so high and i didn't feel like i was working that hard, it's usually due to cardiovascular drift.