(CBS) — It’s no secret that kids are expensive. The U.S. Department of Agriculture estimates that the average offspring comes with a $230,000 price tag.

However, the cost of parenting doesn’t stop when the little darlings leave the nest. As it turns out, the most costly age to have “kids” is when they’re actually adults, according to a new survey co-sponsored by Merrill Lynch Wealth Management and AgeWave, a research group.

In fact, American parents of adult children spend $500 billion annually on their adult children between 18 to 34 years old, or twice the amount they contribute to their retirement accounts, the study found.

The study breaks parenting into three phases — infant/toddler; elementary/high schooler; and adult. And although you might imagine that the most costly phase is when the kids are little and require daycare, diapers and a host of pee-wee-sized products, parents say that’s not true. Just 11% of parents said this was when they spent the most on their offspring.

The toddler phase is the most rewarding and least costly, according to this survey of 2,500 parents and 500 teens. However, it also often spurs a major shift in parents’ working lives. More than half of mothers and 42 percent of fathers took leave from work when they first became a parent. And nearly one in four mothers either switched to a job with more flexibility or started working from home. Fathers more often switched jobs to get better pay or benefits, took on more hours or started to moonlight.

It’s when the kids go to grammar school that costs start to ramp up, with a never-ending need to replace suddenly-too-small clothing; pay for team sports; school supplies and/or private schools and tutors; not to mention the laundry list of teen “must-haves,” that range from iPhones to cars.

This is when peer pressure starts to take a toll, with 69 percent of parents saying that they feel they must provide their kids with the same things that their friends have.

“We live in a world where there is a lot of keeping up with the Joneses,” said Ken Dychtwald, CEO and founder of Age Wave, on a conference call to discuss the findings. The thinking is, he said, “Those kids have a cell phone, so my kids need a cell phone.”

Still, although 44 percent of parents said that these elementary-high school years are the most costly, a slightly higher percentage of parents counter that “you ain’t seen nothing yet.”

Between paying for weddings, college bills, cars and groceries, 45 percent of parents say they spend more than ever on their adult children. After all, the cost of an in-state public college averages $127,000, while private colleges set parents back a whopping $255,000, according to the College Board. But the Merrill Lynch/Age Wave survey notes that parents are kicking in for day-to-day expenses, such as rent, vacations and cable television too.

Indeed, parents collectively spend twice as much on their adult kids each year — an average of $6,600 per year, or $500 billion in total — as they do on elective contributions to their own retirement plans, according to the study. Roughly one in eight provides some sort of financial support to their young adult offspring, according to the study.

“It’s a $500 billion hidden economy,” Dychtwald said. “I started to think this is almost a trade imbalance between parents and kids. Perhaps some can afford to be this generous, but some can’t.”

This can sometimes exact a heavy toll on the parent’s financial lives. Some 72 percent of parents say they put their children’s interests ahead of their own and 63 percent say that’s caused them to sacrifice their own financial security. And more than half say they wish they had set clearer limits.

Not surprisingly, as parenting costs soar, more individuals are thinking twice before they have kids. Only about one third of parents let the cost of having a child affect when they became parents before 1970, when the cost of having a child amounted to less than $30,000. In 2010, three quarters of parents said their financial situation played a role in when they had kids.

— Aimee Picchi contributed to this report.