The eldest son of Queen Elizabeth II has been heir to the throne since his mother became monarch in 1952, when he was 3. He is the longest-waiting heir apparent in Britain's history, overtaking Queen Victoria's son, Edward VII, two years ago.
Charles became a grandfather earlier this year with the birth of Prince George, the first child born to Prince William and his wife Kate.
Palace officials said Wednesday that Charles will contribute the government pension to a charity that helps elderly people.
The future king is entitled to about 110 pounds ($175) per week because of his service in the Royal Navy and voluntary contributions he has made.
As Prince of Wales, he certainly doesn't need the pension fund. He enjoys control of the lucrative Duchy of Cornwall, a vast holding established in 1337 by King Edward II to provide income for his heir. It brings Charles millions each year.
The prince, who is accompanied by his wife Camilla, will mark his milestone birthday representing his mother at the Commonwealth heads of government meeting in Sri Lanka after spending part of the day in India. For decades, the queen has attended the meeting, and Joe Little, managing editor of Majesty magazine, said that Charles saw taking her place as "quite a momentous occasion."
Stepping in for the queen has become more common in recent years as Elizabeth, 87, has greatly reduced her international travel. Her husband Prince Philip, 92, has also reduced his public appearances following a series of medical setbacks.
Over his decades as heir, Charles has become known for his strong opinions on topics such as education, architecture, religion, the environment, organic food and homeopathy.
Little said the prince was enjoying his relative freedom to speak out before he becomes king, a role that will require him to be much more cautious in his pronouncements.
"I think he's very much making use of the time available to him in that he can put forward theories and get things done that he won't be able to do when he becomes king," said Little.
Charles has also celebrated his birthday by serving as guest editor of a special edition of Country Life magazine, which shares his enthusiasm for preserving Britain's rural areas.
Charles used his stint in the editor's chair to upbraid supermarket chains for taking advantage of Britain's small farmers and said Britain's farming heritage is at risk.
In an editorial column, he called the countryside "the unacknowledged backbone of our national identity."
In recent years, Charles has also criticized financial companies which he says focus on short-term gains at the expense of the environment.