Even as he fades from the scene, Mandela at 90 remains a powerful presence for South Africans

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 Friday.

When he turned 89 on July 18, 2007, Mandela celebrated by announcing the …