Oct. 27, 2006  -- Most parents would agree that their kids always seem to grow up too quickly, but now, puberty is hitting young girls earlier than ever.

Nearly half of African-American girls start showing signs of puberty by 8 years old, and some American girls are developing as young as 5, even 4 years old, experts say.

Skyla Jones is an energetic 5-year-old from Georgia who loves to play.

Last year, her mother, Melissa Jones, noticed Skyla was tired all the time, had a body odor, and had blood in her urine.

"I didn't know what was going on," Jones said. "I just went ahead and thought she had a kidney infection. And we went on antibiotics and still it didn't clear it up."

Skyla was eventually sent to Andrew Muir, a pediatric endocrinologist at the Medical College of Georgia, who knew exactly what was going on.

Skyla was menstruating and was experiencing other unmistakable signs of puberty.

"That's what he told me that it was -- that she was having a menstrual cycle," Jones said. "I was really freaked out."

Studies have tracked an increasing trend of early sexual development.

By age 8, almost half of African-American girls and 15 percent of Caucasian girls start developing pubic hair or breasts.

"The switch that normally gets turned on for pubertal development gets turned on too early," said Dr. Diane Stafford of Children's Hospital in Boston.

Until recently most doctors didn't expect to see these signs until age 10.

"It can have many causes," Muir said. "Some are related to genetics, environmental factors, and sometimes we just don't know why it happens."

For Skyla, a thyroid problem caused her symptoms.

While she was too young to really understand what was happening, her mother worried that other kids might.

"I was worried kids would make fun of her because kids are cruel," Jones said.

Because Skyla started treatment for her thyroid problem, all her symptoms have gone away.

Muir expects her next experience with puberty to be at a more reasonable age.