In yet another indication that childhood is lasting longer these days, the number of 20-somethings staying on their parents’ auto-insurance policies after graduating from college is on the rise. About a third of drivers in their 20s are on policies paid for by someone at least 20 years older, presumably a relative. Chubb Corp. reports an 11% increase in the past two years for policies with at least one 21- to 25-year-old driver on another policyholder’s plan. Other big insurance companies confirm that they are seeing an increasing number of parents keeping their adult children on their policies.

This trend accounts for about $4.5 billion of the $149.5 billion auto-insurance industry, according to data from State Farm Insurance Cos., Allstate Corp. and Chubb. Auto Insurers generally allow children to remain on their parents’ auto policies indefinitely, as long as the child is living at home. Most health insurers, by contrast, require that children must be full-time college students to stay on their parents’ policies. At Allstate, 21- to 29-year-old drivers on their parents’ policies represent just 4% of roughly 20 million customers. But that percentage works out to be about 800,000 people, a figure that has steadily risen over the past four years.

Esurance, an online auto-insurance unit of White Mountains Insurance Group Ltd., has found that 34% of all policies with children listed represent children age 23 and older. It is "an increasing trend over time, and given the fact that a lot of folks move back home after graduating, we expect that it will continue," says Kristin Brewe, an Esurance spokeswoman. Kathleen Douglas says she and her husband are keeping their 24-year-old daughter on their auto-insurance policy while she is in school. A Winter Park, Fla., psychotherapist, Ms. Douglas says the couple also subsidizes their daughter’s car payments and cellphone bills. "We want to make sure she transitions well into a paying job," Ms. Douglas says. "Parents are being parents for a longer period of time because kids are being teenagers for a longer period of time."

How much longer? State Farm’s auto-insurance data give a picture: Of State Farm drivers who are 23 to 29 years old, 30% are on the policy of someone at least 20 years older. For 23-year-olds, that level is 54%, and for 24-year-olds it is 45%. The percentage drops as drivers reach their later 20s, but still remains notable – at 19% --for 29-year-olds.

Parents and children today define adulthood differently than they did in the past, sociologists say, with children today reaching adult milestones at older ages than their parents did. Parents also can save money by keeping their children on their policies, rather than reimbursing their children for their own separate policies. That is because most insurance companies offer multicar discounts, which can lower premiums by about 15%. Car insurance for a young driver runs roughly $600 to $900 a year and becomes cheaper as the child gets older.

To keep children on their policies, parents must list them as household residents. College or graduate students away at school only part of the year easily qualify. Children working temporary jobs in other states or actually renting an apartment nearby rather than living under the same roof are a little trickier to claim as members of the household. Christine Tasher, a spokeswoman for Geico, an insurance subsidiary of Berkshire Hathaway Inc., says a child’s claim may not be covered if the insurer found that the child was claiming to live with his parents but actually wasn’t.

YOUNG ADULT MARKETERS!