Prince Hisahito is the first Japanese royal male to reach adulthood in 40 years.

TOKYO (AP) — A notable event in Japan’s imperial history took place as Prince Hisahito celebrated his 18th birthday on Friday. He is now the first male member of the royal family to reach adulthood in nearly four decades, marking a significant moment for a lineage that has held power for over a millennium.

As the nephew of Japanese Emperor Naruhito, Hisahito’s ascent to adulthood raises important questions about the future of the imperial family. His father, Crown Prince Akishino, was the last male royal to reach this milestone back in 1985.

Currently, Hisahito is the youngest member in a family of 17 adults, among which only four are men. His position as the last male heir poses a challenge for a predominately male-driven system that does not permit women to ascend to the throne. Discussions within the government have begun on how to stabilize succession without involving female royals.

The 1947 Imperial House Law, which closely aligns with conservative pre-war family values, dictates that only males can inherit the throne. Additionally, female royals who choose to marry commoners are stripped of their royal titles.

Public sentiment leans toward Hisahito’s older cousin, Princess Aiko, as a preferred future empress. Aiko is the sole child of Naruhito and his wife, Masako, who has a notable academic background as a Harvard-educated former diplomat. However, current regulations prevent Masako from stepping into the role of empress, despite her direct line of descent.

Understanding the succession line can be complicated: Naruhito holds the position of emperor, followed by his brother Akishino as second in line, with Hisahito next in line.

A proposal earlier suggested allowing an empress following Aiko’s birth was quickly discarded once Hisahito was born in 2006. This decision has left questions surrounding the future of the royal family.

In a recent statement, Hisahito expressed his desire to focus on his remaining time in high school. Notably, he has a keen interest in entomology, having co-authored a research paper on dragonflies found on his estate in Akasaka, Tokyo.

A panel of experts, commissioned by the government in January 2022, put forth a recommendation to consider allowing female members of the royal family to retain their titles after marriage. This initiative aims to address the declining number of royals while exploring the option of adopting male descendants from defunct royal families to preserve the male lineage.

Despite these proposals, critics argue that maintaining a male-only line of succession would render any reforms ineffective. Historically, the male-dominated structure relied on concubines to sustain royal lines in earlier centuries.