Instructions
Open the filly's mouth. Look for the first two incisors, or two front teeth. These appear by the time the filly is 2 weeks old. If these are present on their own, you know that the filly is less than 2 weeks old.
Examine the teeth some more, if there are more than two. Look further into the filly's mouth. The second, third and fourth premolars will be present if the filly is 2 weeks old or more. These are collectively called cheek teeth as they are further back into the mouth, level with the filly's cheeks. The filly's teeth do not grow in an even sequence; there will be gaps that need to be filled in over time. So, there will be two incisors at the very front of the mouth, then a gap on either side and then three more teeth on each side of the jaw. There are eight teeth in total.
Count the incisors at the front of the mouth again. If there are four incisors at the front, then the filly is 4 to 6 weeks old.
Look back at the cheek teeth. If the first premolars, also called the wolf teeth, have grown in, then the filly is 5 to 6 months of age. The first premolars will be the forward most premolars in the filly's mouth. There should be a row of 4 cheek teeth on each side of the jaw to make this age estimation.
Age the filly at 6 to 9 months old if there are six incisors at the front of the filly's mouth. The filly now has a complete set of incisors.
Count 24 baby teeth altogether in the filly's mouth, to know that she is a 1 year old.
How to Read the Teeth to Determine the Age of a Young Filly
Assess a filly's age by its teeth. This is an old technique that many horse owners have forgotten. The expression "Long in the tooth" came from the technique of aging horses by their teeth. A young horse is called a foal, and a filly is a female foal. A foal becomes an adult horse after one year. Learning by looking at examples helps, but here is the theory behind determining the age of a young filly.