In the battle between fitness and age, chalk one up for active seniors.

Activity levels are far more important than age in determining fitness, according to researchers at the Norwegian University of Science and Technology, based on a study of thousands of healthy 20- to 90-year-olds.

"Physical condition is the most important factor in describing an individual's overall health, almost like a report card,” said researcher Stian Thoresen Aspenes. In fact, the study showed that 50-year-olds can be just as physically fit was those in their 20s.

It’s the intensity of exercise, the researchers found, that can help reduce the risk of metabolic syndrome, risk factors that predispose people to type 2 diabetes, stroke and cardiovascular problems. Interval training -- short bursts of high-intensity exercise followed by short periods of low-intensity exercise -- is a good way to quickly boost overall fitness, researchers said.