King Charles III inherited the British throne after Queen Elizabeth II died.
At 73, he's the oldest person to become a monarch in the UK, leaving Prince William as his heir.
Royal historian Marlene Koenig says Charles is like a "caretaker" of the throne for the future.
King Charles III knows his reign will last a fraction of the time of Queen Elizabeth II's but will carry out his duty to ensure the throne is there for future generations, a royal expert told Insider.
At 73, Charles is the oldest person to become king in British history. He was only 3 years old when Queen Elizabeth II ascended to the throne in 1952, making him her heir apparent throughout her 70-year reign.
But given his age, Marlene Koenig, a royal historian with a focus on British and European royalty, said the new king is acutely aware of how much time he could have left.
"He knows that his time is shorter," Koenig said, comparing Charles' reign to his mother's. "But he's gonna do the best that he can."
"He is and he knows he is a caretaker for the next generation," she added, referencing Prince William, his eldest son, and Prince George, his eldest grandson. William, 40, and George, 9, are both first and second in line to the throne respectively.
Doing his best in the years he has left is something Charles indicated he's acutely aware of in his first speech as king, pledging to uphold constitutional principles "throughout the remaining time God grants me."
According to Dr. Bob Morris, an honorary senior research associate at UCL's Constitution Unit, all monarchs act as placeholders for their heirs to an extent.
"Somebody's going to replace them," he said. "There's nothing more certain than that."
That said, Morris said given Charles is not a young king, the time he has left could be useful to William and his wife, Kate Middleton. "It'd help to prepare them for taking over," Morris said.
Whether Charles has 20 years left or far less, Koenig said time affords William the opportunity to prepare to be king. She added that Charles's reign also gives William time to be a family man and raise his three children with Kate, without the responsibility of being head of the monarchy.
As it stands, sooner or later, William will be king. And when it happens, Koenig said she thinks he "will certainly have a long reign" in comparison to Charles.