Britain's Queen Elizabeth and her heirs

A handout image released in London on April 20, 2016, shows a Royal Mail postage stamp sheet that has been issued to mark the 90th birthday of Britain's Queen Elizabeth II on April 21, 2016. The stamp sheet, made from a photograph taken in the White Drawing Room at Buckingham Palace in 2015, features four generations of the British Royal family, (L to R) Britain's Prince Charles, Prince of Wales, Britain's Queen Elizabeth II, Britain's Prince George, and Britain's Prince William, Duke of Cambridge. Britain's postal service released a set of special stamps to mark the queen's birthday, including the first to feature her great-grandson, two-year-old Prince George. / AFP PHOTO / Royal Mail / Ranald Mackechnie / RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / ROYAL MAIL / Ranald Mackechnie " - NO MARKETING - NO ADVERTISING CAMPAIGNS - NO SALES - DISTRIBUTED AS A SERVICE TO CLIENTS

Britain's Queen Elizabeth turned 90 on April 21, making her the oldest monarch in the world.

Many official and unofficial celebrations have been held to mark the occasion. A four-day pageant at Windsor Castle has even been planned for May, with more events scheduled to take place in June.