On the blue bar at the top right above the gaint "Stop and read this" banner, there is a link that says "My Settings." Click on that, then on the next screen you will see on the left side another link that says "Edit Your Details." After you click that look towards the middle of the screen under "Email and Password" to find the "Date of Birth" section. There is a drop down selection there that you can use to display age. Make sure you click "save" at the bottom of the page when you are done.
