The millennial on your cellphone plan is ruining retirement

The stereotype of millennials living in their parents' basements is well worn by now, but what about the 30-year-old still on a family cellphone plan? Or the 35-year-old piggybacking on mom and dad's Netflix?

Parental support of adult "kids" ranges from full dependency to steady drips of cash infusions, with all of it degrading retirement savings.

Almost half of U.S. millennials have received some form of monetary assistance, even if they are living on their own, according to a recent study by the financial firm Fidelity.

The top budget item millennials get help with, according to Fidelity: cellphone bills.

The budgetary impact is not chump change. A recent survey from consulting firm Age Wave and Merrill Lynch found that 60 percent of Americans over 50 have provided financial help to other family members in the past five years, with 68 percent of that support going to adult children. The average amount given over that time period: $14,900.

Deciding when enough is enough is a hard task for everyone involved.

"There are so many of us that have enabled our children. It's hard to start saying no," says Mary Ballin, 53, a financial adviser in Walnut Creek, Calif.

"We said: 'When you quit school, by October 1 the phone is on you, and by January 1, the car is on you,' " Ballin says.

One way to facilitate the wake-up call is to participate in a financial literacy program together. Wendy Lawrence got her husband and three stepchildren (ages 17, 19, and 29) involved in one through her employer, SunTrust Bank.

"We had a frank discussion of how much we spend on them. It was eye-opening," she says.

The 29-year-old, who lives at home, now pays rent. Lawrence and her husband put some aside to help him save for a down payment. He also is maxing out his 401(k).

But sometimes that parental wake-up call rings differently. For Barbara Trainin Blank, a freelance writer near Washington, D.C., it makes more sense to pay rent for her 26-year-old daughter to live nearby rather than stay with them because they clash too often.