Six months after Congress and President Barack Obama overhauled the nation's health care laws, some of the first major provisions are set to take effect later this week.

The one change the most people will probably notice first is a provision requiring insurers to cover children until the age of 26.

An estimated one million young adults between the ages of 19 and 25 are uninsured in California today, and many of them will be eligible for coverage on their parents' policies.

And unlike current rules, which require children to be dependents and in school full time in order to stay on their family's coverage, the new rules will apply to most children whether they are in college or not.

A 25-year-old can be on his or her parents' plan even if married and living in another state.

The only definite exclusion will be for young adults who are already covered by a plan at their workplace. In addition, insurance companies will still be able to exclude young adults with pre-existing health conditions, or deny them coverage for that condition.

But another change in the law will prevent insurance companies from denying coverage to children under 19 years old because of their health history.

Another change: insurance policies will have to cover a long list of preventive health measures at no out-of-pocket cost to the policyholder. There will be no co-pays or deductibles for check ups, mammograms and the like.

Health insurers will be able to raise premiums to cover these services, and some have said they intend to do so. Some of those costs might be passed on to policyholders through their employers' benefit plans.

"Most insurers see the value of preventive services and do include them as benefits for their members," said Gerald Kominski, associate director of the UCLA Center for Health Policy Research. "But many insurers do require co-payments or require that (policyholders) count their preventive care toward their annual deductibles. This will no longer be the case."

The new law will also prevent insurance companies from retroactively denying coverage to people based on errors in their applications. People will only lose coverage based on outright fraud, not honest mistakes.

Aaron Keshishian, a 25-year-old student at Cal State Northridge, said Monday that he recently had to choose between health coverage and continuing college, because the only health insurance he could find cost about as much as his tuition.

"Up to my 25th birthday I was covered by my mother's health plan," he said. But as soon as he turned 25, even as a full-time student, his mother received a letter from the insurance company informing her that he would be dropped from her policy.

"For the past three or four months I have gone without any health insurance" he said. "Thankfully I will be covered by her policy until my 26th birthday."