Aug. 31, 2012 -- When it comes to taking motherhood to an extreme, Frieda Birnbaum is unsurpassed -- literally. In 2007, at age 60, she became the oldest American woman to give birth to twins.
Today, at age 65, the New Jersey woman says she's still going strong, chasing after her two five-year-old boys -- Josh and Jarrett -- and also caring for her 12-year-old son, Ari. Birnbaum had her first child when she was 26, but says she feels more energetic now than she did back then.
"I am definitely more energetic," she told "20/20's" Elizabeth Vargas. "We have to redefine age, 'cause it's changed."
Although Birnbaum may be an outlier, it's clear that the average age of motherhood is on the rise. As more women focus on their careers in their 20s and 30s, they are increasingly putting off motherhood, and the overall U.S. birth rate has declined. But women in their early 40s are bucking the trend, seeing the highest birth rates for their age group since 1967, according to the Centers for Disease Control.
Birnbaum and her husband, Ken, first became parents 35 years ago, when she gave birth to their son Jaeson. Four years later, she had her second child, Alana.
After their children grew up, the Birnbaums became empty-nesters -- and they didn't like it.
"I had this biological urge that couldn't be stopped. When I saw a baby, I just wanted to take that baby away and run. You know, hold it and hug it," Frieda Birnbaum said. "As you get older, I've found I have more of an urge to have children than when I was younger."
Birnbaum says she became pregnant naturally in her forties but miscarried. Then she began in vitro fertilization treatments; she will not disclose whether she used her own eggs or donor eggs. At 53, she gave birth to her third child, Ari.
"It was just such a wonderful experience. I said, "You know what? I mean, we could do it again," said Ken Birnbaum.
The Birnbaums decided to pursue IVF again when Frieda was in her late 50s, but there was a hitch. Doctors in the United States refused to perform the procedure because of her age.
And they weren't the only ones voicing disapproval.
"Jaeson was angry at me," Frieda said of her oldest son, who is now in his 30s and has two children of his own. "(He) said we were crazy."
Birnbaum was able to get pregnant with the twins at age 59, with the help of a clinic in South Africa, where she said doctors seemed less concerned about her age. When pressed, Birnbaum admits she "may have" told doctors that she was younger than she was.
Some are concerned with more than just the standard risks facing older pregnant women -- hypertension, pre-term delivery, vaginal bleeding, preeclampsia, and gestational diabetes.
Dr. Arthur Caplan is a bioethicist at the University of Pennsylvania who strongly disagrees with Birnbaum's decision to give birth so late in life. He says it's just not the case that 70 is the new 50. Statistics show that even healthy people will begin to suffer mental and physical decline around the age of 70.
"The central question is what can we do to ensure the best interests of the children. And if you're going to be entering a nursing home when your child is entering junior high school, I think that's trouble," Dr. Caplan said.
Ari Birnbaum says that even today, he feels like his parents don't have enough energy for himself and his younger brothers.
"They seem to get very tired easily," he said.
These days the Birnbaums seem determined to prove the old adage "children keep you young," chasing after their three boys and socializing with parents half their age.
"I'm so inspired by life I am so excited by life," Frieda Birnbaum said. "I am more excited by life now than ever before."