People who want to find out how long they've had their phone often want a phone upgraded. Cell phone companies make offers near the end of a contract term. Many terms last two or three years. In order the keep their customers, the service provider will offer deals several months before the end of the term so that they can sign the customers on for an extended period of time.
Find the phone number of your service provider on your bill. Alternatively, you could check for the number on their website by searching for the name of the company on Google. The website will also be listed somewhere on your bill.
Phone your service provider and ask them how long you've had your current phone for. They will have a record of all transactions available for you.
Check your bill to see how long you've been a customer. Phone companies will have this listed in the fine print. If you bought your phone at the beginning of your term with the company, then you will know how long you've had your phone.