Wendy Prokosch of Brooklyn Park put her three boys on her AT&T family plan when they were in high school.

Ten years later, her boys are now men in their 20s, and all of them are still on the plan, including the oldest, a 26-year-old married schoolteacher.

"We'd put his wife on our plan if they would let us, but we maxed out the number of lines," she said. "Having them on our plan is part of the mom and dad in us, a way to help lower their payments."

Parents think hard about when to give their young children their first cellphone. (The average age is now 11.) Turns out they also think hard about when — or if — to drop their adult children from the family phone plan.

Family plans can be cheaper, but many parents say it's not just a matter of money. They're doing what they can to help launch their children in an increasing complicated, costly world.

More than 40 percent of parents paid for the cellphone service of their 18-to-35-year-old children, according to a Harris Interactive poll done earlier this year.

That number didn't surprise Teresa Swartz, a sociology professor at the University of Minnesota.

"In my interviews, parents see this sort of assistance as a smart and practical thing to do," she said.

Swartz has surveyed hundreds of families about the transition to adulthood for her continuing research study "Growing Up But Not Apart: Young Adults' Relationships With Their Parents."

She's discovered that parents of young adults tend to focus on the big picture, not the Verizon bill.

"Parents observe their kids trying to make it in this economy and think, 'They should struggle for the big things.' Parents want their kids to put their energy into what will launch their careers, maybe taking an entry-level job or unpaid internship. Parents see their efforts and are willing to subsidize a smaller thing."

Swartz said parents recognize that becoming economically self-sufficient is more complex than it was when they were starting out and that they expect to provide assistance to their children for a longer period.

But is it good for young people to float along on the family plan?

"There are a few questions to contemplate when you think about carrying them," she said. "Ask yourself, 'Is anything lost when kids don't have to struggle?' Are you giving them too much? Do they feel entitled to this? Are they grateful for it?"

For many families, the bottom line is, frankly, the bottom line. No matter how you do the math, a family plan is set up to be cheaper than an individual plan.

Prokosch, 45, has a typical plan. She pays $99 plus taxes for her phone service. Subsequent lines, used by her husband and sons, cost $9.99.

"The boys pay their own portion. If they want an upgrade, they pay. They are responsible," she said. "My youngest was texting a friend in Canada and didn't know there was an extra charge for international texting. He ran up a bill of over $100 — and he had to pay it."

Wireless companies bundle services as part of a sophisticated marketing strategy, according to Maria Ana Vitorino, assistant professor at the Carlson School of Management.

"The low-priced multiple lines is how these companies lure you in," she said. "The consumer focuses on the $9.99 per line cost. The company looks at the upcharges — insurance fees, data plan costs, extras for features. Those add-ons are highly profitable. From their standpoint, the more people on the plan — locked in with a contract — the better."

Phone companies also use family plans to lessen turnover and create lifetime customers.

"Consumers develop loyalty to brands," she said. "The companies hope kids that start on these family plans will be with them into adulthood, then stick with them when they select their own family plan."

But it's not all about marketing. Vitorino sees larger shifts in society. A provision in the Affordable Care Act that allows parents to keep children on their health insurance until they are 26 may exert a subtle influence on families.

"Consumers carry concepts from one product or service to the next," Vitorino said. "If the government says you can be responsible for your child for one thing until they are 26, that may give parents license to apply the concept to other services. There is the notion for the kids to be with you longer. There's one change in society and others follow."

Parents who wonder if the kids will ever go it alone might be heartened to hear about Jill Smith.

Smith and her husband, Jason, both 32, have added her parents to their family plan.

"It was our idea," said Smith, of Crystal. "They reimburse us for their lines. They don't text, they don't have a data plan, so they're cheap to carry. After all they have done for us, it's nice to know that we can bring down expenses for them."

Kevyn Burger is a Minneapolis-based broad​​caster, podcaster and freelance writer.