From Samsung's Gear Fit 2 Pro and Gear Sport to the Fitbit Ionic, and of course the Apple Watch Series 3, it seems like every major tech company is coming out with a new fitness tracker or smartwatch these days.

But while the use of fitness trackers is on the rise, certain characteristics—like age and income—determine how likely a person is to purchase and wear one in the first place. People who pick up fitness trackers are likely to use them for at least six months, but those people are also most likely to be young and have higher incomes, according to a new study from the University of Pennsylvania.

In other words: People who use fitness trackers often, and who continue to use them, tend to be fit already. On the other hand, people who could actually benefit the most from fitness trackers often opt not to use them at all.

Overall, 1.2% of the people surveyed use fitness trackers. Look at only young people, though, and fitness tracker usage more than doubles, to 2.8%. On the flip side, only 0.1% of people over 65 activated a wearable device, while people (regardless of age) with incomes less than $50,000 a year made up as little as 0.7% of users. (Researchers analyzed data from 2014-2015 for subscribers to a national health and wellness organization.)

"We found that, though use grew over time, it really varied depending on individual characteristics like age and income," said Mitesh Patel, M.D., an author on the study and assistant professor of Medicine and Health Care Management at Penn.

But whether you're a 20-something gym rat or starting to see those grey hairs sneak in, and whether you're able to spend a lot or a little, fitness trackers aren't going anywhere. And as the technology improves, there's every sign that fitness trackers are becoming ever more reliable tools to manage long-term health. The trick? People have to use them in the first place.