SmartPak has a video about 'how to tell a horses age by their teeth' on their "Ask the Vet Video's"
If you don't have the identification papers of the horse, the most reliable way to know the age of the horse is to look at the teeth.
Horses have baby and adult teeth.
You can roughly tell a horse’s age by which baby teeth they have, and by the growth and wear on their adult teeth.
Most horses will be born with their first set of incisors (the center teeth). Their middle incisors will come in between 4-6 weeks of age, and the outside set of incisors will come in between 6-9 months of age. Horses do have canine teeth, and their baby canines should come in between 6-9 months of age (not all horses have baby canines).
Horses have three sets of baby premolars (3 on each side, top and bottom). They are often born with all of these teeth. Otherwise, they should come in within the first 2 weeks of life.
As horses get older, their baby teeth (caps) start to fall out and their adult (permanent) teeth start to come in.
Horses will start to get their permanent incisors when they are about 2-1/2 years old. The middle set of incisors is the first to grow in. The second set of incisors should come in around 3-1/2 years old, and the outside set of incisors should come in around 4-1/2 years old.
The adult canine teeth come in between 4-5 years of age.
Horses have 4 sets of premolars on each side, top and bottom. The first premolar is very small, and not all horses have this tooth. The first premolar is also called the “wolf tooth.” If present, the wolf teeth usually come in between 5-6 months old. Some veterinarians will pull the wolf teeth at the same time a male foal is castrated. The rest of the premolars don’t come in until a bit later. The second premolar (the first large, flat tooth) comes in around 2-1/2 years; the third premolar comes in around 3 years; and the fourth premolar comes in around 4 years old. (The lower premolars may come in earlier than this.)
Horses have three sets of molars on each side, top and bottom. Some horses may have a very small fourth molar, but this is not very common. Your horse’s molars will be coming in around the same time as his premolars. The first molar comes in between 9-12 months old; the second molar comes in around 2 years old; and the third molar comes in between 3-1/2 to 4 years old.
Hope it helps :)