As he turns 60 today, the Prince of Wales will find a certain historical irony, not only in being the oldest heir to the throne in the long line of English monarchs, but also to have reached what for many people is normal retirement age before taking up the job for which he has spent his life in training. Defining and honing his role as he waits to become King has been an overwhelming preoccupation. It is one that, despite carping, controversy and periods of deep personal unhappiness, Prince Charles has fulfilled with skill, conviction and purpose.

It is appropriate, therefore, that this morning, on his birthday, he will launch Youth Week at The Prince's Trust, the charity he founded 32 years ago, which has proved one of the most enterprising and successful of all the organisations associated with his name. More than 500,000 young people have been trained, mentored and inspired to overcome hardship, fulfil their potential and transform their lives. The trust's work in inner cities is recognised as extraordinarily effective. Its encouragement of volunteering has given purpose to idealists, direction to policy.

There are dozens of other causes to which he has lent support and patronage - ranging from the restoration of the Mary Rose, the Tudor warship whose raising from Portsmouth Harbour he watched 26 years ago, to interfaith dialogue, rainforest projects, the teaching of Shakespeare and the preservation of Britain's architectural heritage. On some issues he has been prescient. He championed organic farming before it was fashionable, and gave warnings of the dangers of climate change and promoted a better understanding of Islam before either became a political necessity. On others he has been obstinate, otherworldly or plain wrong: in his suspicion of science, his dislike of almost all modern architecture and his faith in homoeopathy.

Prince Charles has not shied from vigorous campaigning, lobbying ministers, writing letters and meeting community leaders. He has often sailed close to the political wind, stirring debate and infuriating those who would confine his role to that of the trivial and ceremonial. In all this, however, he has shown a realism about the limits of his “convening power” and an understanding that partisanship permissible in a Prince will have to be set aside in his future role as King.

Unlike his predecessors, the Prince of Wales has had to live out his personal life in the relentless and unwelcome glare of publicity and censorious scrutiny. His failed marriage, his years of unhappiness, quirky convictions and conservative tastes in dress, the arts and architecture have been spread across the world's tabloids and mocked. His views have been misrepresented and his foibles exaggerated. He has, as a result, come across sometimes as an eccentric fuddy-duddy: overly concerned with his own image, out of tune with the mood of his country and ill-equipped to play a properly representative role.

Though sometimes encouraged by his own petulance in not seeking or taking advice, the caricature is unfair. More and more, Prince Charles seeks to be governed by moral purpose. His determination to use his privileged position to expand opportunity and further social causes is very real. So too is his deep and well-informed interest in faith - not just that of the Established Church, which he will one day head, but all faiths, especially their more mystical elements. His commitment to religious tolerance and interfaith dialogue is wholly appropriate for kingship in a world where faith still matters.

Few of his predecessors have forged something positive from the ill-defined of heir: he has made the best of a non-job. Charles has established a pattern that his sons would be wise to follow. It has been no mean achievement.