Also when they lose teeth it is not in any order, although the middle ones tend to wear out first. They usually go very thin (the teeth) beofre they actually fall out, so you have some warning that they are getting old. Often a sheep will struggle when it starts losing teeth, but will do alot better, on a softer diet, when they have all come out and they are totally gummy.
People tend to forget that sheep also have molar teeth at the back of the jaw. When these start to come out, they really have problems. It is also worth checking by running your hand along the side of the jaw, that there are no lumps which indicate abscesses in these back teeth.