There are two aspects to iPhones
The version of the software and which hardware you have. Apple releases an iPhone iteration each year. It also releases a major software update each year (and sadly these days quite a few minor ones). From it's first date of release each handset will get new versions up until it is considered that the new software will swamp the older phone due to the new software features needing more RAM or CPU in many cases. At this point it will stop on the last version it supported.
Apple has a reasonable history on this as mentioned further up. iOS14 is able to be used on something as far back as the 6s which was launched in September 2015. In the real world that is only five years. In the tech/handset/fashion world it is quite a good run.
