As long as you are current based on your age. No need to get one every six months unless you're legally required to do so. As far as the birthday being blacked out, that's only once you reach an interview and actual human eyes are viewing the application. I will assume that the system won't allow your app to even be considered if you're not current. Someone knows better but I'm not sure many/any human eyes actually see your app until the computer spits out your name to be interviewed. There are a couple guys here who are involved in the hiring process who will likely answer more definitively.