Still able to use as a phone.
Still able to run the latest operating system.
Still able to run the newest apps.
Continues to run on an older version of iOS and older apps.
Since the battery can be replaced (by Apple or an Authorized Repair Center), an iPhone could conceivably be used for 10 years or more. But Apple only supports a particular iPhone model with the most current version of the iOS for about 5 years. So if you buy a brand new model after the next iPhone comes out this fall, expect that Apple will keep supporting it with system upgrades through at least 2021 or 2022. Most people would consider this the productive lifespan of an iPhone.