The oldest in the whole world, is Yuriko, Princess Mikasa. Aged 97, she definitely wins this one. She was married to Takahito, the last living brother of Emperor Hirohito. On the picture below, the spry princess can be seen walking beside her nephew, the former Emperor Akihito.
Princess Mikasa was born on 4 June 1923 at Takagi's family house in Tokyo. She is the second daughter of Viscount Masanari Takagi (1894–1948) and Kuniko Irie (1901–1988). Her father was a member of the Takagi clan, formerly lords of the small feudal domain of Tan'an; through her father, she is a great-great-granddaughter of Hotta Masayoshi, a prominent rōjū, or shōgunal minister, during the Bakumatsu period. Her mother was descended from the noble Yanagihara clan, and was a second cousin of Emperor Shōwa; the Emperor's grandmother, Lady Yanagihara Naruko, was Kuniko's great-aunt. Yuriko graduated from Gakushuin Women's Academy in 1941.
On 29 March 1941, Yuriko's engagement to her second cousin once removed, Takahito, Prince Mikasa, was announced. The engagement ceremony was held on 3 October 1941, and the wedding ceremony took place on 22 October 1941. After her marriage, Yuriko was styled Her Imperial Highness The Princess Mikasa. Princess Mikasa frequently visited her husband who was hospitalized during his final months.
Yuriko is a pretty remarkable case. She personally knew several Japanese emperors, including the one who led his nation into WWII. Her husband, the late prince Takahito, was a famously ballsy man who, against all protocol, urged his brother Hirohito to step down as Emperor after losing the war. He later became a professor in Semitic languages and lived to be 100 years old, dying in 2016.
Yuriko is not the only elderly princess in the world. When a Dutch monarch steps down, which they typically do ever since 1890, they revert back to the title they held prior to assuming the throne. So when, in April 2013, Queen Beatrix stepped down from the throne. she became Princess Beatrix. She’s 82 years old and by all accounts in excellent health. I would say she is definitely among the oldest princesses in the world.
Her mother, Juliana, also abdicated the throne. In 1980. When she died in 2004, she was less than a month shy of her 95th birthday. If Beatrix manages to beat her mother’s impressive record, she may be set quite the impressive record.
In England you had the Queen Mother, Elizabeth II’s mum. She made it to a whopping 101 years, but alas, in England former Queens forever remain Queens and do not revert to princess status, so Beatrix wins this one.