You can. Hover your mouse over your name in the top right corner, and click "Personal Details". Then you can change your age.
You actually can't. There's no possibility of changing it yourself. Only mods can do that. Privy, pls.
Ah. Discount all I hath said, for it be untrue. I automatically assumed you could, but I suppose I never knew for sure, as I never needed to do so.
Then you can change your age.