Michael Jackson: 'I Feel ... Very Young' at 50

The Jacksons to King of Pop

The group eventually became known as The Jacksons, and Jackson also had hits as a child solo artist and actor.

Jackson's adult musical career took off with "Off the Wall," and eventually, he became such a superstar that he adopted the moniker, "the king of pop."

But Jackson also developed a reputation for eccentricity. He often was seen wearing a surgical mask in public, and his appearance changed and his skin complexion seemed to grow lighter through the years.

He had a pair of brief marriages, including one to Elvis Presley's daughter, Lisa Marie Presley.

In 2002, Jackson drew criticism for dangling his youngest son, Prince Michael II, then a toddler, from a German hotel balcony as fans looked on. He later apologized.

In 1993, Jackson was accused of molesting a 13-year-old boy who visited his "Neverland" ranch, but settled the case out of court.

Jackson was criminally charged with child molestation in 2003, but was acquitted in 2005.

Is the Thriller Gone?

"Fifty should have been a turning point for him," said the man known only as Phoenix, who once served as a spokesman for the Jackson family and now fronts a tribute band called the Dancing Machine Revue. "The Big 5-0, so far, is not going quite as planned."

After the trial, the family relocated to Bahrain as Jackson fled the media he once courted. Jackson recently returned to the States and has been living in Vegas where he was photographed last month being pushed in a wheelchair while wearing a dreadlocked wig, sunglasses and a surgical mask.

The photo seems to contradict Jackson's own comments that he feels young and can still perform his famous dance moves.

When Jackson biographer Stacy Brown saw it, he thought, "Oh, he's looking for attention again," he said. "Then, I looked closely and said, 'Wow, he's really thin. Maybe there's a problem."

Brown, who was once close to the family and collaborated with Jackson's brother Jermaine and sister Rebbie on books, said Jackson's has a history of physical ailments. "He's battled lupus for years, especially when he's in a high-stress situation. And he's been under that for long time."

He has also battled addictions to pain medications and can put away a surprising amount of whiskey, said Brown, co-author of "Michael Jackson: the Man behind the Mask."

But Phoenix believes it's his emotional demons that are crippling him now.

"He's been under the weather for a while, but I believe it's more or less psychological," he said. "The world has always expected him to be energetic and lively. He has a lot to live up to."

"I think it goes back even further," said Brown. "He had a lot of pressure as a boy, then carrying the group, then being the focal point. It became too much. He let go of more important things in life, his faith. He became a self-worshipper. He let go of his family."

Phoenix, who is still in touch with Jackson's parents and brothers and spoke to Tito just a few weeks ago, said the family would like to celebrate Jackson's milestone birthday with him, but has been unable to reach him. "The family has been trying to get hold of him, it's very hard for them to get in touch with him."

Instead, family members, parents Joe and Katherine and older sister Rebbie, are expected to attend Phoenix's special birthday tribute concert in his hometown of Phoenix. His revue will play its usual mix of Jackson 5, the Jacksons and Jackson's solo hits.

For a while now, the brothers have wanted to reunite with Jackson for a tour. The last one they did together, the Victory Tour, was 24 years ago. A Jacksons tour, Phoenix said, would not only thrill fans but boost the brothers who have struggled financially without Jackson.

But Brown thinks even Jackson touring the way he used to appears to be out of the question. "I don't imagine he's physically capable," he said.

Brown is also doubtful about a new album. "He's been trying to complete an album for a long time," he said. "I think that's within his reach. But will it even really matter? He doesn't even have a record deal. That's what's really amazing. He's in a situation where has to prove himself all over again."

It's a very different situation from his fellow '80s pop icons Prince and Madonna, who also turned 50 earlier this summer.

While Prince and Madonna continue to make hit records, fill stadiums, appear at the Superbowl and on magazine covers, Jackson has become something of a recluse, releasing yet another album rehashing his greatest hits in celebration of his birthday.

"They were on a level playing field for a long time. Then Michael took such a bizarre turn to his life," said Jonathan Cohen, a senior editor at Billboard magazine. "Their careers could not have diverged more."

ABC News' Thea Trachtenberg contributed to this report.