Sarah Tew/CNET

iPhone owners in the United States are apparently no longer in such a rush to upgrade to the latest version of the gadget.

Among people who upgraded from an old iPhone to a new one, the age of the old phones has increased over the past several years, meaning people are waiting longer to upgrade, according to data released Tuesday by research firm Consumer Intelligence Research Partners.

For the 12 months ending June 2013, 34 percent of iPhones were at least two years old when replaced. But since June of 2014, that figure has been hovering at around 50 percent. CIRP based its data on surveys of almost 3,000 Apple customers in the US.

Smartphone makers such as Apple depend on sales not just from new buyers but from current owners who upgrade to the latest model. The notion is that an iPhone owner who gets hooked into the Apple ecosystem is more likely to remain a customer. However, Apple needs to make each new iPhone upgrade-worthy or current owners may just rely on an existing device.

The pace of iPhone upgrades has lost speed for two reasons, according to CIRP co-founder Josh Lowitz.

First, the rate at which innovative, must-have features pop up in each iPhone has slowed.

Second, smartphone buyers used to purchase their devices via subsidized plans that lasted two years. After time was up, you upgraded to the latest model. Now, mobile carriers force you into finance plans whereby you pay the full cost of the phone. The benefit behind such plans is that you can upgrade to a new phone once per year by trading in your existing one. But many consumers may not take advantage of that benefit, according to Lowitz.

"Today, phone financing plans effectively reward customers who have paid for their phone in full," Lowitz said. "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."

Apple declined to comment on CIRP's data.