This was only the beginning of the restrictions now placed on the emporer's daughter, who is banned from marrying a commoner - or she risks losing everything, including her titles and fortune
And this means Princess Toshi may never marry as she is only permitted to marry a noblemen, but there are none left in Japan.
Not only that, but the princess will never ascend to the throne herself as only men can rule.
But Princess Toshi was struggling with life as a royal even before her father became emperor.
When she was just eight years old the princess refused to go to school because she said she was the victim of bullying.
She was eventually persuaded to return to the classroom but only if her mother could go too. It was some time before she felt confident to go to school on her own.
Princess Toshi's parents tried to make her school days more bearable by regularly inviting her classmates to lavish gatherings at the palace.
An investigation revealed that she had suffered "violent things" from boys in another class.
The school, however, explained away the incident, claiming that two boys accidentally collided with her, "which scared her".
In October 2016, there were more worries about the princess when she missed nearly two months from school due to an unspecified illness.
Palace officials confirmed the teenager had been complaining of stomach problems and dizziness, which they attributed to studying for exams as well as practising for an athletics event.
Then in December that year, when official photos were released to mark her 15th birthday, there was shock in Japan at how thin and frail she looked, leading many to speculate she was suffering from an eating disorder.
By the time her time at Gakushuin Girls' Senior High School, the princess was able to spend three months studying at Eton in Britain, staying in a boarding house.
And in the last few years the princess, who loves to play the cello, has been accompanying her parents on royal duties and is said to be planning to attend university.
However, as she matures into adulthood there seems to be little for the young royal to look forward to - as several of her older female relatives have already experienced.
In 2005, Princess Toshi's aunt, Princess Sayako, married a commoner in front of just 30 people.
She had no choice but to relinquish her title and move out of the Imperial Palace into a regular Tokyo apartment.
She was given a dowry of just £1.3million - just a fraction of the £289million a year the royal household lives on.
To prepare her for her new life as a commoner, she had to be taught how to drive and taken to a supermarket to be taught how to shop.
And last year, Toshi's cousin, Princess Ayako, also lost her title and privileges after marrying boyfriend Kei Moraya, who works for a shipping firm.
Ever since she was born, the princess has been surrounded by insanely lavish wealth. There is an army of servants ready to fulfil her every whim and money is no object to whatever she wants.
She has never had to do anything for herself and her father, the emporer, is said to be amazed at how relaxed the British royal family is.
He is said to be stunned that our Queen pours her own tea and serves her own sandwiches, something the Japanese royal family would ever do.
There are now calls to modernise the monarchy in Japan so women can rule and marry outside the nobility.
Princess Toshi would be able to become ruler of other monarchies, such as ours in the UK and in Holland.
In Japan there are just three heirs to the throne, Emperor Naruhito's younger brother Crown Prince Akishino, 53, his son Prince Hisahito, 12, and the emperor's uncle Prince Hitachi, 83.
Experts have warned the royal line could completely vanish if the Imperial House Law is not revised.
In 2005, an expert panel called for a recognition of matrilineal succession and a revision to the law to allow the imperial couple's first born, regardless of gender, to ascend the throne.
But the impetus was halted with the birth of Prince Hisahito in 2006 - the was the first male member of the imperial family born in nearly 41 years.
There are also fears that banning women from marrying outside the nobility, effectively forcing them to leave the royal family if they chose to wed, means the busy schedule of official duties is falling onto fewer and fewer people.
Of the current 18 imperial family members including Emperor Emeritus Akihito, 85, and Empress Emerita Michiko, 84, who no longer perform official duties, 13 are women.
Relaxing the rules is popular in Japan, with 84 per cent of people supporting allowing women to become emporers, not least because it would keep the monarchy relevant in an ever-chaging world.
But Japan's prime minister Shinzo Abe is reportedly against allowing women to rule, believing that because the throne has consistently been passed down through the male line, it should continue the same way.
Meanwhile, Princess Toshi is left in a royal limbo - and facing a stark, lonely future.
Many little girls dream of being a princess - but for Japan's Princess Toshi the reality of being born royal is a very lonley existence.
Princess Toshi, also known as Princess Aiko, is the only child of Crown Prince Naruhito, who ascended the Chrysanthemum Throne to become the next emperor of Japan last year.
At 59, it was a moment he had waited his entire life for but for his daughter, who is now 18, it sealed her lonely fate.
Even on the day of her father's ascencion, Prince Toshi's place had become painfully obvious. She was not at her father's side, neither was his wife of 27 years, Empress Masako.
Women are not allowed to behold the crowning of a Japanese emperor, which is the world's oldest monarchy, beginning in 660BC.