You may be familiar with how dogs age as compared to humans, but what
about the horse? In the first year of life, the foal grows more rapidly that
the human child. From birth until three years, the horse ages approximately
6.5 years for every human year. In his third of life, the horse's age slows
to about five years for every human year. The three-year-old horse is the
rough equivalent of an 18-year-old human. From four years on, the horse ages
only about 2.5 years for each human year. A 22-year-old horse is
approximately 65 years old in human terms. A horse that lives to the ripe
old age of 36 is the equivalent of a 100-year-old person.
    All age equivalents in the charts are appropriate, and just as with
humans, some animals age better than others.

Horse    Humans
1 year    6.5 years
2 years    13 years
3 years    18 years
4 years    20.5 years
10 years    35.5 years
17 years    53 years
20 years    60.5 years
24 years    70.5 years
30 years    85.5 years
36 years     100.5 years