President Obama with his daughters, Sasha and Malia (right). Jewel Samad/AFP/Getty Images

One of President Obama's little girls really isn't so little anymore.

Recent photos of the first family show the Obamas' elder daughter, Malia, now 13, nearly as tall as her parents, who are both about six feet tall.

The president himself joked about his daughter's height during a speech last July in Kansas City, Mo.

"Even though she's five-nine, she's still my baby," Obama told the assembled crowd.

But his baby is growing up, and experts say depending on genetics and the timing of puberty, it's not unusual for some young girls to be noticeably tall.

Genetic factors play a major role in determining height, and since the Obamas are both fairly tall, it's no surprise that their daughter is, too.

But Dr. Gary Berkovitz, chief of pediatric endocrinology at the University of Miami's Miller School of Medicine, said it's difficult to predict a child's ultimate height.

Getting an idea of how tall a child will be, based on parents' height, involves a complicated formula, and might not be an accurate predictor, he said.

"The number you come up with can be two inches more or two inches less than actual height," Berkovitz said.

Another major factor involved in determining height is the timing of puberty.

"If puberty comes early, then there is a growth spurt early and growth stops early, and a child may end up on the short side," Berkovitz said.  "But if puberty comes late, then growth continues."

Predicting height isn't easy, because the timing and length of puberty vary widely.

"How tall a girl is at any point isn't enough to say where she's going to end up," Berkovitz said.  "A girl who's very tall at 12 but who's already had periods for two years is going to stop growing soon, but a girl who's 14 who's never had a period will keep growing."

Estrogen plays an important role in regulating height. Estrogen drives female adolescent sexual development, but also causes the bones to mature and stop growing in both girls and boys.

But pre-pubescent boys shouldn't worry about being shorter than some girls their age. They'll catch up.

"One of the big differences between the height of guys compared to women is that puberty comes later in guys," Berkovitz said.

ABC News' Sunlen Miller and Clayton Sandell contributed reporting.