He wore a trendy black shirt just like many of the kids in the crowd. But Nelson Mandela moved slowly, leaning on his wife and on a white cane as he crossed the stage to adoring cheers.

Public appearances like the one at the London rock concert in honor of his birthday are rare these days for the anti-apartheid icon.

Mandela jokes he has "retired from retirement," but this time it sounds serious. The tall, majestic figure the world saw walking out of prison to freedom 18 years ago is now gray-haired, frail and for the most part silent as he reaches his 90th birthday on Friday.

When he turned 89 last year, Mandela celebrated by announcing the founding of a "council of elders" - fellow Nobel peace laureates, politicians and development gurus pooling their wisdom and influence to tackle global crises. Elders have since jetted to Darfur and the Middle East - but Mandela has stayed at home.

As South Africa's first black president - he only ran for one term - Mandela ushered in a democratic, multiracial society that is still going peaceful and strong.

There are occasional bumps, some sharp. But overall, the Mandela era has confounded doomsayers at home and abroad who doubted South Africa's races could live together under black rule.

After changing his country so profoundly, then turning his energies during his first "retirement" to tackling problems like AIDS, Mandela has left the stage to younger leaders. But South Africa and the world seem reluctant to let him fade into retirement.