Here's something of a shocker. A new study on family phone plans from Harris Poll/NerdWallet says the average age of adults currently on their parents' family plan mobile phone account is 28-years-old.

That's right - 28. That's the average age. And get this - 32% of adults currently on their parents phone plan are 30-years-old, or older.

Also surprisingly, the report says that having an adult child on a family phone in their 20s can actually save the family money, as long as the son or daughter contributes something to the payment.

According to the study, over half (51%) of parents whose adult child is on their family plan say that this arrangement saves both parties money, while 53% said their child pays some or all of their bill. Additionally, 65% of parents who previously had a child over 18 on their plan said that child chose to leave the plan - he was not kicked off by his parents.

Nerdwallet/Harris Poll states that U.S. families can save between $180 and $300 annually, by keeping adults in their 20s and 30s on the family phone plan.

That's great for mom and dad, but do adult children really need to be on their parents mobile phone plan that long?

Evidently, yes.

"I'm 27 years old, and I was on my parents cell phone plan from my high school days until the beginning of this year - a little over a decade," explains Brandon Schroth, a digital analyst at TekBoost, an information technology equipment firm. "Our plan was through Verizon, and it was far cheaper having me on a family plan versus an individual plan."

Schroth says his parents allowed him to stay on their plan, because he was "always responsible" about making payments, and would never go over his data usage or minutes. "Once you've been on a particular plan for years, you become accustomed to its features, including free phone upgrades every two years," he adds. "Another reason I was allowed to stay was because I wasn't making a lot of money at my entry level job after I graduated from college. The economy was not doing well, and my parents felt better about asking me to get my own plan, once I started making a decent amount of money and could support myself comfortably."

Now Schroth is on his own cell phone plan, and feels he learned a lot from being held accountable on his parents mobile phone plan.

"After comparing plans from a variety of carriers, I ended up selecting Sprint's Unlimited Plan," he says. "I have unlimited texts, calls, and data for approximately $85/month. It's an effective plan though because I'm only paying $10 more per month compared to my share of the family plan, and I don't have to worry about data limitations or any additional fees."

As long as adult children follow the trail blazed by Schroth and others, and pay for their fair share of the family phone bill, family experts say it's O.K. to keep them on the plan, even for years at a time.

"You've got more adult children living at home," notes Varda Epstein, a parenting expert at Kars4Kids.com. "A study released in May by the Pew Research Center said that 32% of 18-34 year-olds live with their parents, whereas in 1960, only 19% did so."

Epstein's view is that, since more adult children are living at home, it's important they take as much responsibility for their living expenses as possible, while parents should look for ways to cut the costs of hosting adult children long-term. "So it makes sense to keep adult children on the family phone plan as a cost-saving measure, however, the child should contribute his share of the costs to his parents, to make up the difference," she says."Just as he or she should contribute to rent and utilities and other household expenses."

Kaytie Zimmerman, an expert on Millennial finance, says parents of adult children face a tough decision in determining when their kids should become financially independent. "The days of financial independence at 18 years old are long gone," Zimmerman notes. "Rising college costs, higher student loan burdens, and unemployment and underemployment among recent grads contribute to the drift in average age to self-sufficiency. Young adults are frequently staying on their parents cell phone plans well into their late twenties, sometimes into their thirties, because most carriers don't offer the same rates and plan benefits that they would seek on their own plan."

Zimmerman says the "target number" for each child on going off the family mobile plan is different, based on the individual's current education, career and living situation. "But a general rule of thumb would be to have the adult child separated financially in all other areas, such as health insurance and car insurance, before pushing the issue of cell phone," she adds. "Some providers make it easy to have even non-family members on joint plans with separate payment options to encourage this type of behavior."

Others agree the family phone bill should be the last household bill an adult child should take on for his or her self. "That move really makes sense," says Bob Bentz, president of Purplegato, a mobile marketing agency located in Philadelphia. "After all, keeping one additional phone on a family plan doesn't really cost that much more. But really, the best age to get a child off of your plan is when she gets married - or starts making more money than his or her parents."