His destiny, however, is to be king, a position he will automatically assume with the death of his 92-year-old mother, Queen Elizabeth II. When that happens, Charles will be bound by the constitutional requirement that the monarch refrain from trying to influence policy. Until then, Charles is free to lobby for action on climate change, support organic farming, and fight genetically modified crops as he sees fit.
He's doing all that while increasingly stepping in for the queen and supervising the Prince's Trust, an ambitious charity he founded 42 years ago that has helped hundreds of thousands of young Britons.
Is the candle-crowded birthday cake a signal that it's time for the elegantly greying prince to take it easy? Not on your life, says Charles' wife, Camilla, the duchess of Cornwall. "I don't think he thinks he's 70," she wrote in a birthday tribute in The Telegraph Magazine. "I think it's just a number to him. There's no way that he will slow down. You must be joking. I keep saying 70 is getting on a bit. It's not very old but it is old. You have to slow down a bit."
The royal family is in the midst of a slow, understated transition. The patriarch, 97-year-old Prince Philip, has formally retired from public life, although he makes occasional appearances in support of the queen.
For her part, the queen still maintains a busy schedule, but she no longer makes long haul flights to far flung parts of the 53-nation Commonwealth, and this year she took the unusual step of lobbying the Commonwealth countries to specify that Charles would be the next leader of the group, a position that is not hereditary.
The support for Charles was unanimous, reflecting not only appreciation for the queen's work over the decades but a belief that Charles has a strong commitment to the Commonwealth. Charles has also taken a more visible role representing the queen at some important national events, most recently during the Remembrance Day celebrations honoring Britain's fallen soldiers. He placed the queen's wreath at the foot of the Cenotaph monument while she watched from a balcony seat.
But his working trips abroad and his speeches at home generate precious little buzz as the press focuses on younger, more photogenic royals and their cute offspring. In a way, Charles is sandwiched between generations, caught between his mother, a symbol of dignity and continuity who has reigned since 1952, and his two immensely popular sons, Prince William and Prince Harry, who have along with their wives come to symbolize the future of the world's best known monarchy.
William and Harry also remind many of their mother, the late Princess Diana, who died in a Paris car crash in 1997 after a messy divorce from Charles that for a time tarnished his standing with the British public.
It is William and Harry — along with their wives Catherine, Duchess of Cambridge and Meghan — who appear on the cover of glossy magazines, not the about-to-be-70 Charles. It is the young royals who are seen as glamorous modernizers with the common touch, while Charles is sometimes perceived as dour, preachy and remote.
Camilla says the public doesn't understand how "incredibly kind" and funny Charles is, and William and Harry — taking part in a rare BBC interview to mark his father's birthday — praise the way he has used his undefined position as Prince of Wales to advocate so many important causes, such as environmental protection.
But Harry — who has endeared himself to the British public in part with his impish smile and sunny outlook — urged his dad to cut back a bit on the doom and gloom that often accompanies Charles' pronouncements.
"I would encourage him to remain optimistic because I think it can be very easy to become despondent and negative," Harry said. "But hopefully with his children and his grandchildren, and a few more grandchildren to come, he can get energy from the family side and then carry on his leadership role."
He also had this advice: don't work so hard, and have dinner earlier.