Well, taking it as a riddle, once.

The very first human is the oldest human and they died once.

Where's the calculation? Also this is this would be trivial.

Besides, when human dies he's no longer old. He's dead. His corps might be oldest corpse but he's no longer oldest human.

Either once or about six times a year (guestimating from Google) depending on how you want to interpret that.

It depends on whether you want the oldest person as an individual or as a class. Good riddles shouldn't be forked.

Maybe I was too lax in my description.

This is a math task. Given whatever you need (size of human population, probability of death at any age and similar statistical data) and assuming whatever you need (population distribution is such that it does not change over time or sth. like that) calculate average time between deaths of oldest individual in the world.

In other words instead of:

<google_search> therefore oldest man dies every 6 years.

I want:

<given_this_data_and_assumptions> -> <calculation_and_reasoning> -> oldest man dies every 6 years.