Wearable technology is increasingly popular in the U.S., and its appeal is expected to grow each year. eMarketer’s wearables forecast reports that 39.5 million Americans over the age of 18 used wearables, including smart watches and fitness trackers, in 2015—a 57.7 percent increase over 2014. That number is forecast to more than double to 81.7 million in 2018.

eMarketer defines wearable users as “those who wear accessories or clothing at least once per month that are embedded with internet-connected electronics and exchange data with a manufacturer or other connected device.”

Wearables have found the most traction among adults ages 25 to 44, most with an interest in data and fitness tracking. Older Americans are expected to play a large role in fueling the sector’s growth in the long term, drawn by wearable health monitoring technology. In its forecast period, eMarketer expects adults 65 and older to produce the largest growth rates in adoption.

With market penetration at only 16 percent, advertisers have not yet turned to the technology in large numbers. eMarketer Senior Analyst Cathy Boyle says, “The consensus among the experts we consulted was that advertising will not appear in volume on wearables until one or more of the devices attains significant market share, which makes sense considering scale is a critical aspect of digital advertising.”

Click here for more on eMarketer’s findings.