A few ways: -Ask them when they are inputting in any kind of sign up form for your product (if you have one). Probably best to make it an optional field though

A few ways: -Ask them when they are inputting in any kind of sign up form for your product (if you have one). Probably best to make it an optional field though -Mine the data using facebook (if your clients are on your facebook list) -Send out a survey with some incentive, and with some q's that would gain some knowledge about them plus their date of birth