Paul Bruns is your typical 26-year-old: athletic and ambitious. He's also uninsured. "I definitely think about it when I'm thinking about skiing or snowboarding or doing something else crazy," said Bruns, who's been without health insurance for 10 months. "What would happen if my appendix burst, which is totally reasonable for a guy my age?"

When Massachusetts was crafting a landmark health care law that officially took effect on Sunday, much of the focus was on older residents who typically face larger insurance bills.

But those overseeing the law realized they were missing a key demographic _ young adults like Bruns between the ages of 19 and 26, often in low-paying jobs and strapped with debt.

To entice those like Bruns, the Commonwealth Health Insurance Connector, the state agency overseeing the law, worked with private insurers to come up with young adult plans with monthly premiums as low as $106.

Sunday was the deadline for virtually everyone in the state to be insured, although there is a grace period until the end of the year. Anyone without insurance by Dec. 31 faces the loss of a $219 personal exemption when filing state taxes next year.

One of the toughest challenges is simply convincing younger people they need insurance.

"If you're young and healthy, it's not at the top of your mind," said Larry Levitt with the Kaiser Family Foundation, a California-based nonprofit health policy foundation. "Young people frankly don't think about catastrophic coverage and don't think about access to the system for routine coverage."

About 27 percent of Massachusetts residents ages 19 through 26 are uninsured, compared with 10 percent of the under-65 population as a whole, Levitt said, citing U.S. Census figures.

The young adult plans, from $106 to $220 per month, are available for anyone aged 19 through 26 who makes more than $30,630 annually, doesn't get health insurance through work or school, isn't still on a parent's plan, and isn't qualified for subsidized care.

They are being offered by the state's top insurers including Blue Cross Blue Shield of Massachusetts, Harvard Pilgrim Health Care, Fallon Community Health Plan, Tufts Health and others.

"These are designed to be affordable," said Dick Powers, a spokesman for the Connector.

But even the low cost can be a burden on some young people, said John McDonough, executive director of Health Care For All, a group that backs universal and comprehensive health care.

"If you're living paycheck-to-paycheck, you're paying off student loans, paying rent, you're in a different situation from someone who isn't already burdened by debt," he said.

The problem isn't unique to Massachusetts.

There are about 13.7 million people in the 19-to-29 age bracket in the U.S. without insurance, according to the New York City-based Commonwealth Fund.

It's the fastest and largest segment of the uninsured population, said Sara Collins, Commonwealth Fund assistant vice president lead author of "Rite of Passage? Why Young Adults Become Uninsured and How New Policies Can Help."

"This age group does tend to be healthier than the population at large, and per capita spending on health care for this age group is about $1,500 per year," she said.

The state has tried to educate the 19-to-26 age bracket with a series of newspaper and television ads and opinion pieces submitted to college newspapers. One print ad featured a photo of a young woman bungee jumping off a bridge. A TV ad has run during Red Sox games, hoping to reach roughly the same demographic.

Not everyone is convinced. Critics say anyone who exceeds the annual maximum benefits _ which can be as low as $50,000 _ may find themselves deep in debt.

"Many of these young adult plans aren't real insurance," said Andrew Cohen, of The Access Project, a Boston-based health care resource center affiliated with Brandeis University. "The real reason for insurance is to protect you from financial burden, and these plans don't do that."