It is among the top early selling points of the health-care overhaul — a new rule that has particular appeal for middle-class, middle-age voters: Young adults who lack health insurance will soon be able to remain on their parents’ plans until age 26.

But although Obama administration officials note that the provision will help millions, the benefit is proving less immediate than many families expect.

The administration’s success in convincing dozens of insurers to comply with the provision earlier than the law requires has left many parents with the impression that their adult children will be eligible for continuing coverage far sooner than is likely to be the case, experts said.

According to the law, health plans don’t have to comply until their first renewal date after Sept. 23. For some plans, that’s as soon as October. For many, it’s January. For others, the compliance date won’t be until May.

That means many college seniors whose parents’ health plans allowed them to stay on until they graduated this spring may face a gap in coverage until the new requirement kicks in.

Soon after the law was passed, Health and Human Services Secretary Kathleen Sebelius called on insurers to close the gap by complying with the […]