At 28 or 29, Kim Jong Eun, the presumed new leader of North Korea, will be the youngest person ever to have authority over a nuclear arsenal.

At that age, neuroscientists say, the brain is still in the final stages of development. Developmental psychologists say there’s a good reason that America’s Founding Fathers set 35 as the minimum age for a president: People younger than that just don’t have the experience or skill to deal with complex decision making. Historians warn that the track record of young leaders is weak. The Bible puts it bluntly in Ecclesiastes: “Woe to thee, O land, when thy king is a child. . . .

“Kim Jong Eun is not the same man that he will be in 10 years, or even in five years,” said Sam Wang, a Princeton University neuroscientist and author of “Welcome to Your Brain.” “The ongoing maturation we all have observed in people in their 20s is reflected in changes in brain structure.” The connections in the frontal part of the brain “are not quite done growing and developing. The frontal parts of the cortex are important for restraining impulses and making long-term plans.”

In the ranks of world leaders, the age-old reliance on old age — or at least middle-aged maturity — remains very much the rule.

Most world leaders these days take office at age 50 or older; President Obama was unusually young when he was sworn in at 47.

But there are a dozen or so rulers who came to power before age 30. About half are sons who succeeded their fathers. Their performance, today and through history, is less than impressive.

When England’s King Edward VI acceded to the throne in 1547 at the ripe age of 9, he had been raised by his father, Henry VIII, as “this realm’s most precious jewel.” Lavished with toys, the child king — who died at 15 — displayed a savage temper: He ripped a live falcon apart in one fit and later had his uncle beheaded for taking too active a role in governing the country.

One of the youngest leaders of a country today, King Mswati III of Swaziland, was 18 when he was crowned in 1986. He has reneged on more than $10 million in grants to AIDS orphans, even as he has amassed a massive personal fortune, according to the International Monetary Fund. Mswati’s response to his nation’s staggeringly high HIV infection rate was a five-year ban on girls younger than 18 having sex. During the ban, the king took a 17-year-old wife — his ninth — and then fined himself a cow for violating his own edict.

The value of humility

Reporting on North Korea amounts to a journey back to the days of Kremlinology, when Western intelligence services resorted to studying photos of public gatherings to try to suss out the power structures in isolated, secretive communist countries. Until recently, the only photo the CIA had of Kim was of a high school boy in shorts, said Jerrold Post, who was director of the CIA’s Center for the Analysis of Personality and Political Behavior before pioneering the study of political psychology at George Washington University.

In the young Kim’s case, his sudden ascension this week after the death of his father, Kim Jong Il, comes at a very early phase in his preparation for power. “He has not had the opportunity to win the loyalty of those around him,” Post said. “There has to be a lot of uncertainty in the military and the political structure about whether he can handle the pressure and responsibility. This is very different from when his father took over; he had been designated very early and was systematically groomed.”

It’s not clear whether Kim Jong Eun’s uncle, Jang Song Thaek, will act as an informal regent to the new leader, but South Korea’s intelligence service is predicting that some sort of council of elders will share power with the young Kim.

Of course, immaturity and impulsivity are not exclusive to the young. Kim Jong Il, who succeeded his father in 1994 at age 52, was given to a kind of excess often associated with the young and immature; he wore platform shoes and a bouffant hairdo and was for a time, according to the Hennessy distillery, the world’s largest buyer of its top-of-the-line cognac, Paradis.

This is a golden age of sorts for young leaders, especially in the world of technology, where Facebook alone has produced three billionaires younger than 30. But even among creative young technologists who make a bundle with start-ups, “they tend to make a big splash and then sell the company,” said David Bjorklund, a developmental psychologist at Florida Atlantic University and author of a book arguing that immaturity is an essential phase of discovery and not a time when the young should be considered “adults in training.” “With very rare exceptions, people in their 20s are not the managers or chief executives. The skills you want in a chief executive are more those we associate with wisdom — people who can live with contradictions.”

Gifted people in their 20s excel and even dominate in fields such as technology, sports, music and math — “fields where you can take information at face value, where there are absolutes and not a lot of shades of meaning,” Bjorklund said. “But putting Mark Zuckerberg in charge of the State Department would probably not be the best idea.”

A key indicator of the maturity that lends itself to good decision making is what psychologists call “metacognition,” the ability to know what you do and don’t know.

“Most of us who function well probably think we’re a little more competent than we truly are, but we aren’t trying to run a country,” Bjorklund said.

That sensitivity to our own limitations is not something that has been greatly in evidence among North Korea’s ruling family, Post said, but it is a crucial skill for someone trying to solve problems in a country of deep and abiding deprivation. “The lack of empathy for the lower-level citizens among the North Korean leadership is really striking,” Post said.

In the field of leadership training, humility emerges as a strong indicator of the ability to make wise decisions, said Tim Elmore, president of Growing Leaders, an Atlanta nonprofit group that conducts leadership training programs for colleges, businesses and sports teams. “In my 20s, I thought I was the deal,” he said. “Even at 52, emotions can fog my ability to make good decisions. A mature leader of a business and especially of a country can’t be selfish and must seek wise counsel, and that’s not something we’ve seen from North Korea’s leaders, even when they were older and more mature.”

Getting ready to cope

From the earliest days of recorded history, the immaturity of young leaders has led nations to appoint regents, elders who can guide and advise the monarch. But even young leaders who have been prepared since birth for the responsibility they will someday take on often act immaturely during their early years in office.

“It’s a real hazard of coming to power at an early age,” Post said. “Look at Bashar Assad” of Syria, who succeeded his father when he was 34 and is now directing a harsh crackdown against anti-government protests: “Is he feeling impelled to show he has the toughness to do the job? The question for Kim is: Does he have that seasoning, that experience, that sense of knowing how far he can push? The evidence indicates he has been pampered to the extreme. There’s no reason to believe he has the wisdom to understand the magnitude of the problems he faces.”

The few details that have emerged about Kim’s life to date show little indication that he has had to make decisions under pressure, and “being raised with too little stress is bad,” said Wang, the Princeton neuroscientist. In studies of rodents and primates, stress hormone responses go sky-high in those who never experienced stress as youngsters. If Kim wasn’t raised facing “any stress at all,” Wang said, “he hasn’t developed coping mechanisms.”