If you look a little old for your age, that doesn't necessarily mean you have poor health, a new study finds.

The finding challenges a long-held idea, said researchers from St. Michael's Hospital in Canada.

"Physicians have simply assumed that their quick assessment of how old a person looks has diagnostic value," Dr. Stephen Hwang, a research scientist at St. Michael's and an associate professor at the University of Toronto, said in a statement. "We were really surprised to find that people have to look a decade older than their actual age before it's a reliable sign that they're in poor health."

Ninety-nine percent of people who look 10 or more years older than their age have poor mental or physical health, the study found. [Related: Images Show How Exercise Affects Aging ]

The researchers had 126 people ages 30 to 70 fill out a questionnaire that scored their physical and mental health. Then they took photos of the patients and gave them to 58 doctors to assess how old the patients looked. Doctors then guessed how healthy the patients were.

The doctors were unable to predict the health statuses of patients who looked up to five years older than their actual age . But doctors were able to accurately guess the health statuses of patients who looked 10 or more years older than they were, the study said.

Doctors sometimes make snap judgments about patients' health based on whether they look old or young for their age, Hwang said. But they shouldn't assume that people who look young are healthy and people who look just a little older than their age are not, he said.

The study was published Nov. 5 in the Journal of General Internal Medicine.