By Nicholas Witchell
Royal correspondent, BBC News
As monarch, Charles would be expected to be much less outspoken
He has reached the age at which most people retire. Yet Prince Charles is still waiting to undertake the role for which he was born.
Indeed, Charles, who is celebrating his 60th birthday, has been the heir to the throne for so long that he is now the longest-serving monarch-in-waiting in British history - and there is the prospect that it could be many more years before he finally gets the chance to achieve his birthright.
No-one who knows the prince has ever suggested that Charles would wish to hasten the only circumstance which will give rise to his succession.
The Queen will never abdicate. Period. And even if this 82-year-old monarch were to become incapacitated in some way, she would remain on the throne, albeit with the sovereign's powers transferred under the Regency Act of 1937 to Charles.
But he would be Regent, not King.
So Charles will only succeed to the throne when his mother dies. And while, plainly, that is not a development he wishes to hasten, he could be forgiven for feeling a degree of frustration at the way his life has played out.
Despite his occasional petulance and the tangled imperfections of his private life, all the evidence of the past 40 years suggests that Charles is a fundamentally decent man who has spent his adult life striving to make a difference for the better in Britain.
He believes he has a duty to use his position as Prince of Wales to bring together people of wealth and influence to focus on some of the major issues which face us
He could, as he said on the BBC television documentary "Charles at 60: The Passionate Prince", have sat back and done very little other than enjoy the immense privileges and almost unlimited pleasures to which a Prince of Wales is privy.
Without question, Charles does at times revel in the privileges - to a sometimes extravagant degree - and he has had his share of the pleasures, but he has also devoted a considerable amount of time and thought to trying to prove that he is worthy of the special position in life that he gained purely by his birth.
The Prince's Trust was his idea and, in the 32 years of its existence, it has helped more than 500,000 disadvantaged young people to improve their lives.
That alone would entitle him to credit. But Charles has involved himself in very much more than the Prince's Trust.
He has spoken out about the environment (with undoubted prescience), inter-faith relations, GM foods, architecture, education, holistic medicine, and much more.
Some of his interventions have earned him rebukes from politicians and others who have grown tired of his frequent expressions of concern, either in speeches or in the so-called black-spider letters written in his sprawling long-hand.
Certainly his outspoken position on the danger, as he sees it, of GM crops, and his sympathy for Tibet over what has been done to it by the "appalling old waxworks" who lead China, have caused genuine concern within government that this is a prince who lacks the self-restraint and discretion that might reasonably be expected of someone in his position.
Lobbying and campaigning
Charles, of course, sees it differently. As he said in the BBC documentary, he regards his interventions not as "meddling" but as "mobilising".
He believes he has a duty to use his position as Prince of Wales to bring together people of wealth and influence to focus on some of the major issues which face us. This is a role which he describes as his "convening power".
And despite the reservations which some feel about his activities, Charles' officials say he is not going to quieten down.
He still wants to make a difference and he is going to continue, as they put it, to use his position to "enthuse, inspire and warn" to the best of his ability.
There is, though, one very particular danger of the route down which Charles has felt compelled to go for the past 40 years.
After so much lobbying and campaigning on the issues which matter to him, will he be able to stop when he becomes King?
That has been the so-far unanswered question. But in the BBC documentary, we gained our first insight into how Charles himself approaches this issue.
He was asked whether, as King, he would still "champion big themes".
His answer was instructive and is worth recording in full.
Charles said: "I don't know. I don't know - probably not in the same way.
"But I like to think perhaps that after all this, eventually people might realise that some of the things I've been trying to do aren't all that mad and that I might have some 'convening power' that could be put to use."
Charles appeared to be saying that, as King, invitations could well be going out from Buckingham Palace (or Windsor Castle) to attend what might be described as "The King's Conference" on, say, the future of the rainforests, the merits of GM crops, or even the plight of Tibet.
It may yet be many years before he gets there, and when he does his reign may be comparatively short, but on the basis of what he said in this week's BBC documentary, Charles' reign may be one of unusual interest.