Wow! Congratulations on your phone lasting you that long, OP!
I'm not sure about an "average lifespan" of phones, but anything older than a year/year and a half is often a good indication that something *may indeed* go wrong with it tech-wise. My G3 was about eleven months old when it started giving me a lot of issues.
Sounds like you're taking very good care of your phones - good on you! Are you looking at the G6 as an upgrade?