The queen's birthday is marked twice a year, once when she was born (in April) and once on the "official" birthday in June, when there is a parade.
The royal family celebrated the day on social media by sharing photos of the Queen through the years.
Now the oldest reigning monarch, she dedicated her life to service on her 21st birthday. More > https://t.co/mcVCkOE7UY #QueensBirthday pic.twitter.com/chSdUWfARJ
— The Royal Family (@RoyalFamily) April 21, 2017
To mark The Queen's 91st birthday, we are sharing this photo from 1952 of Her Majesty and The Prince of Wales. #HappyBirthdayHerMajesty pic.twitter.com/ednBFygYue
— Clarence House (@ClarenceHouse) April 21, 2017
Check out the gallery above to see how Queen Elizabeth II has changed from her days as Princess Elizabeth, to ascending the throne in 1952, to today.
PHOTOS: Prince William and Princess Kate