I think your horse is over 20. See that groove on the teeth? That appears at the gum line at around 10, extends half way down the tooth by 15, all the way down at 20, is half way gone at 25 and completely gone at 30. Since it starts at the gum line and goes toward the grinding surface of the tooth, it looks like the groove is less than half way gone from gum line to grinding surface, so he would be over 20 but less 25 as it doesn't seem to be half way gone. Another indicator would be to look at the shape of the grinding surface of the incisor teeth. The shape changes from oval, to triangular and after 20 to rectangular. The tooth that is circled is indeed a canine, indicating that he is a he (male)...most mares do not have them and those that do are typically much smaller than those of their male counterparts.