There are many reasons why the maximal heart rate decreases with age.
The most prevalent is the heterogenous thickening of the walls of the heart.
Cardiac output is stroke volume times heart rate.
When the wall thickens, the heart needs more energy to achieve the same pumping (bigger stroke volume) or it needs to increase heart rate at rest.
Because of physical limitation, the stroke volume cannot be increased.
Heart needs to also rest which it does before the action potential reaches the AV conduction system.
Note the long time just after the action potential reaches AV node.
However, the heart cannot increase its maximal heart rate if the elasticity of the walls have decreased in ageing.
Ageing often leads to heterogenous thickening of walls of the heart.
If this is the case, then T wave gets high fast; which causes the risk of bleeding when exercising.
Doing sport or just feeling tired can cause a long-standing pain in the heart area in the chest.
Sport then again leads to dynamic hypertrophy of the heart tissue, which increases the maximal heart rate.
Ageing then again does not have such a dynamic changes.
There are secondary reasons caused by ageing
- hormonal changes
- decreased metabolism
- decreased physical activity
which are associated with decreased maximal heart rate.
Why most endurance athletes reach their peak cardiovascular performance at around age 30?
Human's best age is at about 25 years old physiologically.
Muscles grow in rest.
When practicing at that age, recovery time of muscles is short.
It takes long time to improve cardiovascular performance (1-5 years), since heart muscle improves slowly.
I think the slow hypertrophy of heart musculature and the best physiological age of human are the reasons why the most successful endurance athletes are about 28-30 years old.