Looks like incremental improvements are taking their toll on those who were formerly some of the most eager upgraders: iPhone users.

According to a new report from Consumer Intelligence Research Partners (CIRP), iPhone owners are waiting longer to upgrade their devices, causing the average age of retired iPhones to increase steadily.

From the June 2013 quarter to March 2016, CIRP found the percentage of retiring iPhones over three years old increased from 5 percent of all phones to 12 percent. Additionally, from June 2013 to March 2016, the percentage of iPhones either under a year old or 1 to 2 years old dropped from 66 percent to 51 percent.

Overall, CIRP partner and co-founder Mike Levin said the average age of retiring iPhones has increased by around three months over the past three years.

Fellow CIRP partner and co-founder Josh Lowitz said the firm determined iPhone upgrade timing has slowed for two reasons. First, Lowitz said the extended upgrade cycles correspond to a slowdown in the rate of change in iPhone features. Second, Lowitz said new financing plans from the carriers have encouraged iPhone owners to keep their devices longer.

“Only a few years ago, mobile carrier phone purchase subsidies encouraged customers to upgrade a phone on a steady two-year cycle with no monthly discount for holding a new phone beyond two years,” Lowitz said. “Today, phone financing plans effectively reward customers who have paid for their phone in full. This motivator appears to outweigh the impact of any early upgrade options that carriers offer with the new financing plans, and the increased availability of trade-in programs and used phone purchase websites.”

Since new iPhones in the United States are increasingly sold to current iPhone owners, Levin said the longer cycles will have a significant impact on future iPhone sales figures.

“In the absence of compelling new features, which will encourage early upgrade from iPhone 6 models, these factors will influence and probably limit U.S. iPhone sales in the coming quarters, relative to past quarters,” Levin said.