David Cameron: I grew up in my brother's shadow
David Cameron lived in his brother's shadow while he was growing up, he writes in a column for the Big Issue.
The PM, who guest edited the latest edition of the magazine, said his older brother Alex had been a role model but he always felt "a few steps behind".
He said it was not until he left school he felt he was making his own way.
He also said his gap year travels to the Soviet Union showed him the "greyness of life under Communism" and contributed to his political awakening.
In the 25 July edition of the magazine, Mr Cameron writes about his brother Alex - who was three years his senior at Eton and is now a lawyer who heads his chambers in London.
"I lived in the shadow of my older brother. He was three years older, went to the same school, and was a huge success, on the sports field and almost always lead actor in the school plays.
"It was great to have that kind of role model, and I was incredibly proud of him, but like many younger brothers you find yourself always a few steps behind.
"If I could give my younger self some advice, I'd say: don't worry about it; your life is not predetermined; you'll find your own feet in your own way. It was not until I left school that I felt I was breaking out of my brother's shadow and doing my own thing."
He also paid tribute to his "strong family" and his father, Ian, who died last year, adding: "A lot has been written about my background, but the great privilege of my upbringing wasn't just the wealth, it was the warmth. We all got on, we were all there for each other, there was so much love and support."
And he said his travels around Eastern Europe after leaving school, which included visiting the former Soviet Union, had played a part in his political awakening.
"I will never forget the greyness of life under Communism, the lack of choice, freedom and expression," he added.
"I began to develop a political consciousness, a sense of what was right and wrong. In particular, the importance of freedom and the state being there to serve people, not be their master."
Big Issue co-founder John Bird is a supporter of Mr Cameron's "Big Society" policy to devolve power to communities, which critics say is a cover for cuts.
The magazine, launched in 1991, is sold by people who are homeless or in temporary accommodation as a way of earning an income.
Mr Cameron said it was a "fantastic example of how we can reduce dependence on state hand-outs" which was a "natural fit" with his own political philosophy.