Part One of Two:
Actually most people in a viking-age world would die as children.
In pre modern societies more than half of all the persons born died in the first few days, months, or years of their lives.
In a modern society, people can die at any age, but most die during what is considered old age.
In a pre modern society, people can die at any age, but the probably of dying at a specific age is much more equal at various specific ages than it is a modern society, and there is a very big spike in the death rate in early childhood.
And no doubt you can look up charts showing the percentage of the population who died at various ages in different societies.
But you should remember, the next time you see a teenager and think how young and inexperienced they are, and that their life hasn't really started yet, that actually that teenager has already lived longer than the majority of all humans who have ever been born.
And the next time you see some little child, whose life seems to have just begun, remember that child has already lived tens of times longer than the entire lifetimes of many millions of humans, and even has lived hundreds of times longer than the entire lifetimes of many millions of humans.
So forget about everyone living what you consider normal length human lifespans in a viking-age society with primitive medicine. In a viking-age society, only the minority of humans who live to be adults will then have a more than fifty percent probability of living to what you consider normal old age.
In real life, some humans have always lived tens, hundreds, and even thousands of times as long as the humans who lived the shortest life spans.
So if the longevity blessed people in your would live to be twice as old as normal old age -100 to 160 years - or three times as old -150 to 240 years - and Character A wonders how it could be possible for someone to live twice or thrice as long as other people, Character B could ask Character A how old they are, and then remind Character A of Character C, a toddler in the village who died last week, and point out how many times the entire lifespan of Character C Character A has already lived, a difference many times greater than the two or three times Character A was wondering about.
Medically and biologically, that would be a somewhat poor analogy, since there should be different reasons for the extra longevity of the longevity gifted than for the vastly different ages that normal humans die at.
But if there is magic or religion in this viking-age setting such an analogy should be enough to convince Character A that the extra longevity of the longevity gifted is not such a great difference from the vast range of life spans among normal humans.
So it is possible that most of the human characters in your setting will accept that the extreme longevity of some exceptional characters is a more or less random statistical variation or will have some commonly accepted magical or religious explanation - correct or otherwise - for it.
Part Two of Two:
If there is a genetic component to the longevity, it is possible that your society will not understand it well.
You may have read that Meroveus or Merovech, the legendary ancestor of the Merovingian dynasty of the Franks, was alleged to be half human. His mother, wife of King Clodio, was allegedly raped by a river monster which took the form of a bull, and Meroveus allegedly had webbed fingers and toes.
With modern genetic understanding, I assumed that the half human nature of Meroveus would be that he got half of his genes from his human mother and half of his genes from his river god father.
But in pre modern times, a common theory was that all of a person's genes came from the father, and the mother nourished the fetus but didn't give it any genes. And another theory was if two men had sex with a woman at almost the same time they might both contribute genes to the child that resulted. So in medieval times Meroveus was assumed to be half human because it was believed that half his genes came from the human King Clodio and half from the river god, and none of his genes came from Clodio's wife.
Since a pedigree was later invented tracing the Merovingian dynasty to the royal family of ancient Troy, the belief that Meroveus got half of his genes from King Clodio meant that people (like Emperor Maximilian I) who claimed descent from Meroveus could also claim descent from the royal family of Troy.
Similarly, the half human Minotaur didn't get the human half of his genes from his mother Pasiphae; instead half of the Mintaur's genes came from the human King Minos and half from the bull, who both had sex with Paisiphae.
So maybe your society would believe that all of a person's genes came from their father, and there was no need for a long lived man to mate with a long lived woman in order to pass on their longevity to their children.
Assume that a man becomes King of Sarmtonia age 30 when he still looks like a little boy and lives to be 1,120 years old, in the 1,090th year of his reign. He takes the title of Emperor of Sarmtonia during his reign. The Emperor of Sarmtonia and all of his descendants marry women of ordinary longevity, and the men of each generation have half the longevity of their father's generation.
The Emperor of Sarmtonia He might give birth when aged about 400, and thus in the 370th year of his reign, to his oldest son and heir, who gets the title of King of Kings of Sarmtonia.
The King of Kings of Sarmtonia might give birth to his oldest son, when aged about 200, in about the 570th year of his father's reign, and the son would get the title of King of Sarmtonia.
The King of Sarmtonia might give birth to his oldest son, when aged about 100, in about the 670th year of his grandfather's reign, and the son would get the title of Duke of Sarmtonia.
The Duke of Sarmtonia might give birth to his oldest son, when aged about 50, in about the 720th year of his great grandfather's reign, and the son would get the title of Count of Sarmtonia.
The Count of Sarmtonia might give birth to his oldest son, when aged about 25, in about the 745th year of his great great grandfather's reign, and the son would get the title of Lord of Sarmtonia.
The Lord of Sarmtonia might give birth to his oldest son, when aged about 25, in about the 770th year of his great great great great grandfather's reign, and the son would get the title of Younger Lord of Sarmtonia.
And at 25 years per generation, there could be about a dozen more generations of male line descendants born during the reign of the first Emperor of Sarmtonia.
The First Emperor of Sarmtonia's heirs would start dying with the later generations dying first until they reached ordinary human longevity.
The Count of Sarmtonia should die aged about 70 in about the 790th year of the First Emperor's reign.
The Duke of Sarmtonia should die aged about 140 in about the 810th year of the First Emperor's reign.
The Lord of Sarmtonia should die aged about 70 in about the 815th year of the first Emperor's reign.
The younger Lord of Sarmtonia should die aged about 70 in about the 840th year of the First Emperor's reign.
The King of Sarmtonia should die aged about 280 in about the 850th year of the First Emperor's reign.
The King of Kings of Sarmtonia should die aged about 560 in about the 930th year of the First Emperor's reign.
And when the heirs die with the younger generations dying first, the Sarmtonian royal family might assume that the now deceased royal wives of ordinary longevity had affairs with men of ordinary longevity, so that each royal descendant got half of his genes from a royal man and half from a man of ordinary longevity, so that the royal longevity genes were diluted by half in each generation and each generation had half the longevity of the first.
So the Sarmtonian royal family might then adopt an Islamic type system, where the monarch and princes kept their wives and concubines guarded by eunuchs in harems to make certain they would not have affairs with any men and dilute the royal genes any further.
Even though what they should do according to modern genetics would be to seek and marry long lived women.