OK, there might be some confusion here...
There are 2 different things in your Account management: your personal info (which is under the "User Info" tab, and your BABY info (under the "Baby Info" tab). I'm guessing you were referring to the birthday field under Baby Info, which is for your children, and set so that you can only enter children's ages up to 10 years back. (Actually, I think we are changing this to 18 years).
In fact, I don't even know where you would edit your age - was it even required on registration??
Anyway, the Account Management page still requires a lot of work - it's still a bit sloppy right now, working a bit like a Frankenstein's monster with different bits hastily sewn together. Once we can devote more resources to it, we'll get all this cleaned up. (that's one of the main reasons why we still call this Forum "beta"!)