Prince Hisahito, second in line to the throne and the nephew of Emperor Naruhito and son of Crown Prince Akishino, turned 13 on Friday.
Prince Hisahito is the only heir of his generation, as the Imperial House Law states that only males descendants of emperors can ascend to the throne. Females, including his elder sisters and the emperor's 17-year-old daughter, Princess Aiko, are banned from doing so.
The young prince, currently a first-year student at Ochanomizu University Junior High School, visited Bhutan last month with his parents on his first overseas trip.
The family's exchanges with Bhutanese King Jigme Khesar Namgyel Wangchuck and his family during the visit were widely reported both in Japan and Bhutan.
It is common for future emperors to visit other countries and experience other cultures. Both the emperor and his younger brother have studied at the University of Oxford.
In late April, there was an incident in which knives were found on Prince Hisahito's school desk, just as Japan was preparing for the abdication of Emperor Akihito at the end of the month.
A man arrested within days of the discovery told investigators he intended to stab the prince, and criticized the imperial system and way of imperial succession.
The government is seeking to launch discussions on ensuring stable imperial succession amid a shrinking number of royals performing official duties, including whether to allow female succession. Currently, female members must leave the imperial family if they marry a commoner.
But conservative members of Prime Minister Shinzo Abe's ruling Liberal Democratic Party remain opposed to female succession.