AP

The portion of young people without health insurance has been declining since a rule was enacted in 2010 allowing children 26 years old and under to stay on their parents' health care plans, a new Gallup poll shows.

In the fourth quarter of 2012, 22.7 percent of Americans ages 18 to 25 reported having no health insurance, according to the survey, down from 24.5 percent in the beginning of 2012. Gallup has been tracking health insurance coverage rates since January 2008. The percentage of young people uninsured reached its highest level in the third quarter of 2009, at 28.7 percent.

While the portion of young people without insurance has declined, the levels of uninsured among older people remained about the same, Gallup found. Americans ages 26 to 64 are more likely to uninsured now than they were in 2008 or 2009, most likely because of economic factors like the unemployment rate.

That may or may not change next year, when some of the Affordable Care Act's most significant reforms go into effect, including the mandate that all Americans acquire insurance or pay a fine. Additionally, each state by next year is expected to have a health insurance exchange up and running -- an online marketplace through which consumers can comparison shop for insurance.