1. Look at the teeth. A horse's teeth are constantly growing, and they develop certain characteristics as they age. For example, the cups in the front teeth (incisors) gradually wear down and disappear as the horse gets older. The dental arcade also changes with age, becoming wider as the horse grows older. A qualified equine veterinarian or dentist can estimate a horse's age based on its teeth.
2. Check the body conformation. As horses age, their body composition changes. For example, their shoulders become less upright, their croup becomes more sloping, and their belly may sag. The body may also become thinner or more pot-bellied.
3. Look at the coat. A horse's coat can change color and texture with age. For example, a dark horse may become lighter with age, and a glossy coat may become duller.
4. Observe the behavior. Older horses may be less energetic and playful than younger horses. They may also be more reluctant to move or be groomed.
5. Ask the owner or veterinarian. If you know the horse's history, you can ask the owner or veterinarian for its exact age.
It's important to note that these are just some general guidelines, and there is always some variation between horses. For the most accurate estimate of a horse's age, it is best to consult with a qualified equine veterinarian or dentist.