See photos of Queen Elizabeth through the years in honor of her 91st birthday

Happy Birthday, Queen Elizabeth II! The longest-reigning British monarch turns 91 years old on April 21, 2017.

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.

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