There will be many raising a glass to Prince Charles as he reaches this milestone birthday, not least amongst the ranks of those helped by his many charitable enterprises.
But amid the celebrations, there is an undercurrent of uncertainty about what the future holds for both him and the monarchy.
Charles acknowledged as much last week with his assertion that when he is King, the meddling in politics that has been a troubling feature of his life so far will cease. “I’m not that stupid,” he said, a little testily, in the BBC documentary broadcast to mark his birthday.
No Sir, it’s plain that you’re anything but stupid. But the fact that the issue needed to be addressed at all is symptomatic of a sense of unease about what happens to the monarchy when the Queen’s extraordinary reign finally draws to a close.
She has never put a foot wrong. Charles, on the other hand, has frequently put his foot in it. It’s one thing for the heir apparent to badger Government ministers and express views about matters including technology, architecture and homeopathy, but it’s quite another to have a King whose loudly-expressed views might be at odds with the people over whom he reigns.
Doubtless the Queen has deeply-held opinions too, and in recent years there have been glimpses of what she is thinking behind that inscrutable façade, notably when former Prime Minister David Cameron let slip that she was delighted the people of Scotland had voted against independence.
Overall, though, she has been wise enough to keep her counsel, resolutely staying out of politics and recognising that if the monarchy is to have universal appeal to the people, it must not take sides.
That has been one of the key reasons why republicanism has never gained any real traction during her reign. She has made – and kept – the monarchy an institution for all her subjects rather than letting it be swayed by any outside influences.
It has won her the respect and admiration of people of all political persuasions, and because of that any debate about the role and relevance of the monarchy has been muted.
But when she is gone, that may change. Charles, for all his undoubted sincerity and commitment to causes in which he believes, cannot possibly enjoy a comparable level of respect.
There is nothing inscrutable about him. The agonies of his marriage to Diana, and the subsequent heartache over whether the country would accept his marriage to Camilla Parker-Bowles, were all played out in public.
He has felt it necessary to bare his soul, even to justify his actions, and the willingness of his two sons to talk freely about their inner lives has inevitably shone light on his role as a father.
This is a very different approach to the business of monarchy than the one that has proved so enduring for the past 66 years, and it remains to be seen if it has anything like the same staying power or appeal to the people.
The picture that has emerged is of a complicated man who is strangely naïve and unworldly. There is little sign of his mother’s shrewdness. It is entirely believable when former prime ministers, both Labour and Conservative, say that the Queen’s wisdom was invaluable to them, but difficult to imagine the same being said of Charles.
The question mark over how popular King Charles will be was emphasised by the delicate diplomacy it took to persuade Commonwealth countries to accept him as the Queen’s successor as head of the organisation.
Some plainly had reservations, and appeared to be swayed only by the Queen’s publicly-expressed wish that they should agree. It was the respect and affection in which she is held that carried the day, not Charles’s own qualities.
And he has many. He was ahead of his time in expressing concern about damage to the environment and the work that the Prince’s Trust has done over decades in helping young people make the best of their lives deserves nothing but praise.
As the longest-serving heir apparent in British history, Charles has had abundant time to learn what it takes to be sovereign. The causes he has championed have been born of his sincere belief that he is working for the benefit of the country and its people.
He cannot be criticised for attempting to create a useful role for himself beyond that of monarch-in-waiting, but what the legacy of his passions will be when he is king is impossible to predict.
Happily, there is no sign of that happening any time soon. The monarchy remains in the safest of hands. And as he turns 70, Charles might usefully reflect that there is still much to learn from his mother about being circumspect.