Car insurance is expensive, and insuring young adult drivers adds to the cost. But families do have options to manage the burden. Unlike health insurance, a parent’s auto insurance policy has no required age cutoff for children. (The Affordable Care Act allows children to remain on a parent’s health plan until age 26.

After that, they must have separate coverage.)“It’s not an age question,” said Mark Friedlander, a spokesman for the Insurance Information Institute, an industry group. Rather, young drivers’ circumstances, such as where they live or whether they are students, come into play when considering separate coverage. Douglas Heller, director of insurance at the Consumer Federation of America, an association of nonprofit consumer advocacy groups, agreed that there was no blanket answer. “It’s more situationally specific,” he said.

The question has been cropping up more often: A growing number of young adults live at home with their parents as they confront higher living costs and struggle to pay their student loans. Here’s what to know, in general, about insurance for young adult drivers. Some details may vary by state and carrier, experts said, so it’s always best to confer with your insurance agent.

Why are insurance rates higher for younger drivers? Drivers in their teens and early 20s have less experience behind the wheel and are at a higher risk for serious accidents. That is why insurance premiums are highest for drivers in those age groups and generally become more affordable by age 25, said Julia Taliesin, an economic analyst with Insurify, a rate comparison and shopping site. Inclusion on a family’s policy is often the most affordable option for young adult drivers, she said, because they can benefit from their parents’ lower risk rating and potentially from other factors, like their parents’ stronger credit history. (Most insurers consider credit-based scores when setting auto rates, arguing that they correlate with the likelihood of filing a claim.

Seven states bar or restrict the practice.)The average monthly premium for two parents with two cars that carry full coverage — liability, collision and comprehensive — is $243, according to Insurify data. Adding a 20-year-old who lives in the home to the policy increases the premium by an average of $210, to $453. But if a 20-year-old had a separate policy, the average stand-alone premium would be $311.

What if a child is a student? Students living at home are generally eligible for coverage under their parents’ car insurance policy. Typically, all licensed drivers 16 or older living in a household are listed on the policy. It’s the policyholder’s responsibility to notify the insurer of eligible family members.

One possible exception: If the students are living at home and buy their own car and title it in their name, some insurers may require a separate policy unless a parent is added as a co-owner, Mr. Friedlander said. If those students go away to college and take one of the parents’ cars with them, they are still considered a member of the household and can remain on their parents’ policy. That includes students attending graduate school — even extended programs, Mr. Friedlander said. “You could have a 40-year-old in the house” who is covered by the family policy, he said.

Students who don’t take a car with them to campus can also remain on the parents’ policy. “They come home for holidays and during the summer,” Mr. Friedlander said, and are covered during those periodic visits. Are there discounts for students living on campus? Yes. Families should ask their insurer if they are eligible for a so-called away at school discount if their child goes to college without a car, said Marguerita Cheng, a certified financial planner in Gaithersburg, Md., and a pa...