This photo, released by North Korea's state media on Monday, shows Kim Jong-un, the country's leader, speaking to officials during his inspection of a chicken factory at Hwangju County in North Hwanghae Province, North Korea, the previous day. Kim turned 40, Monday, without any fanfare. Yonhap

While some experts had suspected that North Korean leader Kim Jong-un's birthday would be different this year, he turned 40 on Monday without any public celebration or recognition.

Since rising to power when he was still in his 20s, he is considered by some to have established a stable leadership over the past 12 years. But North Korea still does not acknowledge the day of his birth, Jan. 8. In contrast, those of his predecessors ― father Kim Jong-il and grandfather Kim Il-sung ― are usually celebrated as national holidays with concerts, fireworks and sometimes even military parades on a grand scale.

In their Monday and Tuesday reports, North Korean state media made no mention of Kim's birthday, which was first disclosed by Kenji Fujimoto, Kim's former sushi chef . Although the date is known to be accurate, the year is less so, with some saying it could be 1982 or 1983, not 1984. The website of South Korea's Ministry of Unification shows Kim Jong-un was born on Jan. 8, 1984, as widely known.

So why is it still a secret? Some analysts say Kim may think he is still considered too young and has achieved too little to hold lavish birthday festivities to match those of his ancestral predecesors.

Yee Ji-sun, an expert on North Korea's society and culture at the Korea Institute for National Unification, a state-funded think tank, believes the main reason for the silence may be the story behind his birth and his late mother, who was born in Japan.

"Once his birthday is designated as a national holiday, it would inevitably bring attention to his mother, which I think one of the biggest political weak points for Kim Jong-un," she told The Korea Times on Tuesday.

Ko Yong-hui, Kim's mother, was born in Osaka's Koreatown on June 26, 1952, where her father worked in a sewing factory run by the country's Ministry of War. She, along with her father and some half-siblings, moved to North Korea in 1962 as part of a repatriation program when she was 10. In the early 1970s, she began working as a dancer at the Mansudae Art Troupe, which led to her personal relationship with Kim Jong-il, that of a consort.

There were some reports suggesting Kim Jong-un was seeking to idolize his mother soon after taking power. But such efforts are no longer visible. According to experts, this is because of North Korea's "chulsin songbun" — or caste classification based on family background — which ascribed her the lowest social status as a "hostile," one of those born in Japan.

"When Kim Jong-un tries to highlight the legitimacy of his power, he tends to emphasize his connection to his grandfather more than his father. That's probably because of the same reason," Yee said.

Surrounded by veteran officials and military officers, Kim Jong-un might be still conscious of his relatively young age, she said. "That's another political risk given that he has no personal story like his grandfather, who once fought against Japanese occupiers, or a state-crafted, fabricated story for his father," she added.

But it is possible that Kim Jong-un's birthday will later become official. Kim Il-sung's birthday was designated as a national holiday in 1968 when he turned 56. Kim Jong-il did so for his own birthday when he turned 40.