Phones are pretty indestructible from age, except for the battery. I still have a Nexus 4 lying around and it still works after 12 years. I'd say it's similar to how most electronic devices last an eternity, as long as they don't get wet, hot or dropped.

Edit: And the buttons. Those die way too quickly but if you're lucky they can last a long time too — especially if they aren't used much after a few years.