She is expected to spend the day at the estate's Wood Farm cottage, a personal sanctuary where she also spent her first Christmas since Philip's death in April 2021.
Philip loved the cottage, in part because it is close to the sea, she said in February when hosting a rare public event at Sandringham.
"I think the Queen's approach to birthdays very much embodies her keep calm and carry on attitude," said Emily Nash, the royal editor at HELLO! magazine.
"She doesn't like a fuss."
This birthday comes during the Queen's platinum jubilee year, marking her 70 years on the throne. While Thursday will be low-key, public celebrations will take place in June with four days of festivities.
The day marks yet another milestone in a tumultuous period for the monarch, who has sought to cement the future of the monarchy amid signs of her age and controversy in the family.
After recovering from a bout of COVID-19 earlier this year, the Queen's public appearances have been limited by unspecified "mobility issues".
Prince Andrew's multi-million pound settlement with a woman who accused him of sexual exploitation also caused unwanted headlines for the royal family.
But the Queen got an early birthday treat last week, when grandson Prince Harry and Meghan, the Duchess of Sussex, paid her a joint visit for the first time since they stepped away from frontline royal duties and moved to California in 2020.
Harry, in an interview with NBC, said his grandmother was "on great form," though he added that he wanted to make sure she was "protected" and had "the right people around her".
Britain's longest-serving monarch, the Queen has spent much of the past two years at Windsor Castle, west of London, where she took refuge during the pandemic.
