That's not true. iOS updates, contrary to popular belief, do not intentionally slow devices down. iOS re-indexes the entire OS and migrates libraries which does cause the perceived slowdown for about 3-4 days after a big update, but after that, it's back to normal performance.
Also, all devices do tend to slow down over time. Newer software on older hardware = slower performance compared to newer devices with the latest hardware. It's just the natural evolution of tech. All of the new features in newer versions of the OS will usually not run as well on older devices as newer devices that had the new features designed around the new hardware.
For anyone wanting to read more on this topic, here you go:
http://www.imore.com/iphone-slow-explained-truth-about-ios-updates