I always tell my clients that 1 year of life on a cell phone is equal to 40 human years, so your phone is reaching 160. Congrats! There are two tests that your phone needs to pass in order to answer your question on the average life of a cell phone: 1) Does the phone work as well as I need it to work? and 2) Does my cell phone do everything that I need it to do?
Not everyone needs the latest and greatest phone, despite all the hype. If the answer of the two questions are 'Yes', then you're good to go. If not, upgrade.