Horse Years Methodology

Accurate mapping of horse age to the equivalent human age requires a methodology. The methodology developed by Dr. Doug Stewart is based on developmental milestones (physical, sexual and psychological) and summarised in the following table.

Attribute Horse Age Human Age
Can walk First day 1.5 years
Can run First day 2.0 years
Normal vision 4 months 5 years
Sufficient maturity for initial minor training 1 year 5 years
Early emotional independence 1 year 8 years
Two-thirds adult weight 1 year 13 years
90% adult height 1 year 13.5 years
Sexually immature, but capable conception 2 years 13 years
80% adult weight 2 years 13 years
Start of adolescent behaviour and emotions 2 years 13 years
Sexually mature but still maturing physically, psychologically, emotionally 3 years 17 years
Mature (physically, psychologically, emotionally) 4 years 20 years
Average lifespan if well treated, no major injuries, no major illnesses 24 years 70 years

Table 1. Equine and Human Developmental Milestones

Horse Age 0

Humans are highly unusual in the animal kingdom, in that development stages which occur while still in the fetus stage in other animals, occur after birth in humans. For example, most animals can walk shortly after birth, whereas people do not walk until 18 months of age or run until two years old. A horse at birth (or very shortly thereafter) has approximately the same physical capabilities and age as a two year old human.

One can also observe in a horse early behaviours (e.g. search food, flee danger) which are similiar to that of a two year old child, although in the case of a horse it is more instinctive whereas in a child there is a greater cognotive element.

There are of course significant differences between a new born foal and a two year old child. For example, a two year old child has greater language capability, but as a horse will have only limited language capability even at maturity, this is not relevant in terms of calculating comparable maturity levels.

On balance, a foal at age 0 has approximately the same relative maturity as a child of age 2.

1 Year Old Horse

This period is the most difficult one to map horse age to an equivalent human age.

  • In terms of training, it needs to be short and simple (e.g. raise hoof for farrier work, the horse equivalent of a child learning to tie its shoes), roughly the equivalent of a child at age 5.
  • In terms of emotional independence from its mother and mixing with the wider social group, the 1 year old foal has approximately the same development as a child aged 8.
  • In terms of physical size a horse is two-thirds body weight and 90% adult height, which is the same maturity as a child aged 13. However, this is just physical size maturity and not physical maturity in the wider sense (e.g. a 1 year old horse cannot bear offspring but a 13 year old child can). In terms of the non-size elements of physical maturity a horse at age 1 would be closer to that of an 8 year old child.

Consequently, depending on which measure one uses, the equivalent child age is between 5 and 13 years old. Balancing the various different indicators indicates an approximate maturity level equivalent to an 8 year old child.

2 Year Old Horse

A 2 year old horse has a maturity level (sexually, physically, behavoiurally) which matches that of a 13 year old human.

3 Year Old Horse, 4 Year Old Horse

A horse at age 3 has a maturity level that matches that of a 17 year old child. The different measures (sexual maturity, physical size, and emotional behavioral development) all provide a consistent mapping.

Likewise, the various measures show a strong maturity match of a 4 year old horse to that of a 20 year old adult.

Horse Age 5 and above

The liespan of a horse depends on a range of factors. A well-kept pony may well live to 40 years, but many working horses do not live to 20 years. The average lifespan for a reasonably well kept horse is around 24 years. In developed countries, the average lifespan for a person is seventy-some years. Therefore:

  • After reaching maturity (age 4), an average horse will live about 20 more years (to age 24).
  • After reaching maturity (age 20), an average person will live about 50 more years (to age 70).
  • This indicates that 20 horse years equals 50 human years, or 2.5 human years per horse year.

The use of 2.5 human years for each horse year (after age 4) is supported by further detailed examination of the developmental process. Supporting maturity factors include maximum aerobic capacity and maximum muscle bulk. Supporting aging factors include menopause and ability to bear or sire offspring.

Horse Years to Human Years

The following table shows for each horse age the approximate equivalent human age. It also shows the relative rate at which the two mature (e.g. the first year of a horse's life, it matures the equivalent of 6 human years; after maturity each year a horse ages the equivalent of 2.5 human years).

Horse Age Equivalent Human Age Human equivalent Change
0 (newborn) 2 years  
1 year 8 years 1st horse year = 6 human years
2 years 13 years  2nd horse year = 5 human years
3 years  17 years 3rd horse year = 4 human years
4 years 20 years 4th horse year = 3 human years 
5 to 24 years 21 to 70 years  After 4th year, each horse year = 2.5 human years 

Table 2.  Equine and Human Age Maturity Equivalents

Caveats

The above information and tables are largely based on averages. In practice, the rate at which horses mature and then age varies substantially from horse to horse. Consequently, the information provided here should be considered indicative averages only. 

One difference between the way horses age and the way in which humans age, is that horses have a much higher early mortality rate. In developed countries (e.g. USA, UK), human infant mortality and human child mortality are very low. However, a substantial number of horses die in infancy and childhood, which reduces the average expected lifespan. To illustrate the difference:

  • A human child aged 0 will have an expected livespan of approximately 73 years, while a human child aged 10 will have an expected lifespan of 74 years (another 64 years from its current age).
  • In comparison, a horse aged 0 may have an expected lifespan of 24 years but a horse aged 6 may have an expected lifespan of 28 years. The difference is allowing for mortality during the first 6 years.
  • The implication of this is that as a horse gets older, it is passing the relatively high early mortality stages which are part of the horse breed average, and consequently is becoming more likely of substantially exceeding the breed lifespan average.
  • Another way of saying this is that a young horse is likely to live only to the expected breed lifespan, but an older horse may well exceed the average lifespan by a substantial amount.

Another difference between horses and humans is the difference between the average livespan and the maximum lifespan. In humans the average age is about 74 and the maximum recorded is 122, which means that the oldest person in the world has lived 65% longer than average. The average horse age is about 24 years but the oldest is 62 years, which is 158% more than average. Even if one takes a long-lived breed of 35 years, this is still 77% more.

In summary, many horses die well before or live well after their horse breed average. For further discussion, see