The world's oldest woman celebrated her 115th birthday on Tuesday in a Japanese nursing home with her favourite mackerel sushi dish on the menu.Misao Okawa, a descendant of kimono merchants in the city of Osaka, western Japan, told a broadcaster she had never expected to live to such a great age but had managed it "thanks to everybody.”Late last month Okawa received a certificate from Guinness World Records confirming her status as the oldest living woman. Japan, known for the longevity of its people, is now home to the oldest man and woman on the planet.Okawa was born on March 5, 1898 and married in 1919. She had three children, two of whom - a son and a daughter - are still alive and in their 90s, Japanese media reported.The centenarian, who has four grandchildren and six great-grandchildren, has reportedly never had any major health problems but broke her leg at 102 years old.The world's oldest living person is a 115-year-old Japanese man who lives in Kyoto, not far from Okawa. He will turn 116 next month