North Korea's Kim Jong Un believed to have turned 40

This picture was released on Monday of Kim Jong-un and his daughter, but no mention of the birthday was made

North Korea's Kim Jong Un has hit the big 4-0 - or at least, that is what people suspect.

The leader of the secretive state has never actually revealed his birthday, leaving North Korea watchers to puzzle out when it actually takes place.

If they are right, he hit the milestone age on Monday, the same day official pictures were released of him and his daughter visiting a chicken farm.

The low-key day stands in stark contrast to those of his predecessors.

In fact, the birthdays of his father Kim Jong Il and grandfather Kim Il Sung - large, national celebrations during their time in power - are still marked by holidays to this day.

Exactly why the date, suggested to be 8 January, remains a secret is not known. It has been hinted at previously, with North Korean officials thanking then US President Donald Trump for his birthday wishes back in January 2020 - although they did not specify which day.

Further evidence of the date had emerged six years earlier, when US basketball player Dennis Rodman sang happy birthday to Kim Jong Un in front of a crowd of thousands.

Gilsup Kwak, head of One Korea Center, a website specialising in North Korean affairs, suggests a number of reasons for the silence - not least, his mother Ko Yong Hui, and the fact she was reportedly born in Japan.

There is still widespread distrust of Japan - which colonised the region between 1910 and 1945 - in North Korean society, and the link could undermine Kim Jong Un's bloodline claims, meaning it has so far been swept under the carpet. Making a big deal of his birthday, therefore, may bring unwanted attention.

"Kim Jong Un's Achilles' heel is the truth of his birth," Mr Kwak explained.

However, there could be numerous other explanations, he says, including believing "that idolising his ancestors and family is enough" and wanting to "promote his image as a humble leader".

Others suggest his age is the problem. Experts say that older members of North Korea's ruling elite feel he is still too young - although it is notable that his father's birthday became an official holiday the year he turned 40.