From 12-19 months the central incisors replace the front milk teeth, hence the sheep is a "2 tooth" or hogget.
The next two teeth appear after 12-19 months and the sheep then becomes a "4 tooth". It becomes a "6 tooth" after 23-26 months, until at 28-48 months it becomes an "8 tooth" or Full Mouth. Sheep have no teeth in their upper jaw which consists of a dense, hard, fibrous pad.
A "broken mouth" sheep is one with a full mouth where teeth are broken or missing,this makes it difficult for the sheep to graze effectively, particularly where the grass is short.
If you see a sheep in poor condition, when the other flock members are not, the first thing to do is check the mouth for obstructions,abscesses or broken teeth.