By Susan Johnston NEW YORK ( MainStreet) --The recession forced countless millennials into boomeranging back home with their parents to save money. But even some of those adult children who are gainfully employed, financially independent and perhaps even engaged or married still piggyback on the family cell phone plan. In fact, 29% of parents of 18- to 35-year-old children continue to pay their cell phone bill after their kids move out of the house, according to a Harris Interactive survey of 620 parents for the Wall Street Journal. Kristin Serio, a 24-year-old account executive in Orlando, Fla., is part of this trend. Her grandparents paid her college phone bill through college graduation, when her parents added her to their plan, which includes unlimited data, talk and web. Serio says she'll eventually start her own family plan once she gets married or her three-year-old son is old enough for a cell phone. "But until that happens it's very nice to not have to worry about the bill ," Serio said. Susan Newman, a New Jersey-based social psychologist and author of Nobody's Baby Now: Reinventing Your Adult Relationship with Your Mother and Father (Walker and Co, 2003), says this dynamic is common. "While they have financial independent children, it's still a way for them to feel like they're taking care of their kids," she says. "One of the issues is how complicated these cell phone plans are. In some respects, it's less expensive and a lot easier just to keep your son or daughter on your plan." Still, that's not to say that sharing a family plan is without complications. A mom blogger made headlines in December for having her teenage son agree to an 18-point contract in order to get an iPhone. Although Newman says that parents of adult children probably shouldn't have their child's smartphone password or review the list of calls, they should discuss who pays for overages in advance. After all, exceeding data caps or shared minutes can prove costly. "As soon as you have a cell phone plan, you need to understand your data plan no matter who's paying for it," says Wendy Weaver, a certified financial planner with Bethesda, Mar.-based wealth management firm FBB Capital Partners. "It's a good lesson you can use to teach your children about financial literacy."