The Royal Family's official Twitter account shared a black and white photo of the monarch at just one month old, being held by her mother Elizabeth, Duchess of York, at her christening – an impressive 91 years ago.
The account also shared a speech made by the Queen on her 21st birthday, in 1947, in which she says: "I declare before you all that my whole life whether it be long or short shall be devoted to your service and the service of our great imperial family to which we all belong."
Although April 21 is the Queen's real date of birth, she also her celebrates her official birthday on the second Saturday in June. She generally spends her real birthday in private at Windsor Castle, however royal gun salutes will be fired today in the U.K. to commemorate the occasion.
"Queen's Birthday" and "Happy Birthday Your Majesty" have been trending on Twitter as well-wishers have taken to social media to bid the longest-reigning monarch, who celebrated her Sapphire Jubilee in February.