Reading reviews of Shine a Light, Martin Scorsese's film on the Rolling Stones, is fascinating, especially if you spent your most rewarding parent-taunting moments playing the music of such obvious bad boys.

The reviews are spattered with adjectives such as wrecked, ancient, wizened, wrinkled, withered, simian, arthritic, decrepit, sagging. The easily shocked Sunday Times reviewer found the glimpse of Sir Mick's (taut) naked stomach and (tiny) hips too much and reported wanting to roar: "Yuck, leave it out, Grandad!" Under this brazen ageism are two questions. First, the awesome spite of an impatient younger generation wanting to know: how can they be so old - and get away with it? Second, and more important: what keeps them going when they are already rich and successful?

The film makes the answers abundantly clear. They are shamelessly, blissfully in their element making music and performing. As Keith Richards says, "The thing is, we love what we do." The Stones have found their life purpose, so why ever stop?

I work with many apparently successful clients who have not found their life purpose. They report a feeling of lost opportunity, of wasted talent and an overwhelming sense of unease with themselves. My client "Simon" will stand for many.

Simon had entered his public sector organisation as a fast-stream graduate and looked destined for a yet more distinguished career, possibly ending with knighthood. Yet he felt he was on a conveyor belt that carried him forward, regardless of his own wishes.

Constantly comparing himself with others at his grade, Simon felt like an impostor. Sunday nights were his lowest point as he contemplated yet another draining week. He was bored and aware he might be using alcohol to stave off the difficult questions he had avoided for so long.

When you face this kind of predicament, the place to start is not, as so many do, with "Who else will have me?". It is to ask: "What would I do for sheer pleasure even if no one paid me?"; "What strengths do people consistently describe?"; "What is unique about me?"; "When have I experienced that mixture of challenge, joy and exhilaration that athletes call being 'in the zone'?". Answering these questions virtually always reveals a consistent thread. Ignoring any mismatch is what leads to unhappiness.

In Simon's case, the solution was to pay attention to his religious beliefs and his ability to make things happen through impeccable logistics. As a young man he had wanted to enter the ministry but had been dissuaded by a dominant father. Simon did a deal with his organisation, first training as a priest and then working part time before leaving to run a significant slab of his denomination's organisation at money far below his previous salary.

Mick Jagger has another lesson, too. He has definitely not had Botox or a facelift and his locks are not cunningly chestnut-dyed. He is as campily, cockily, annoyingly and prodigiously talented as ever. Daring to be who you really are in the face of pressure to conform is all part of the secret.