Horse Years

I had someone ask me today what is a horse's equivalent human age for  intelligence? Would they be considered as smart as say, a two-year old child? I had to really think about that.  I'd never had the question phrased quite that way. I'd have to say I wouldn't gauge a horse that way.
It got me thinking about 'dog years': it's common that we equate one dog year as seven human years, so a dog who is two years old would be the same as a fourteen-year old child. Some say it's more like 15 years to one, with smaller dogs having a longer life span than large dogs. Some get into detailed calculations, like adding four years to every year after age two. For example, a three-year-old dog is equivalent to 28 in human years; a four-year-old is 32, a five-year-old, 36, a six-year-old, 40--and so on.
(Read more: How to Determine a Dog's Age in Human Years | eHow.com http://www.ehow.com/how_2088154_determine-dogs-age-human-years.html#ixzz1InSpXRhs)
For dogs though, it's more a measurement of physical growth than mental growth. Mentally I'm guessing that dogs may be like men and remain fun-loving kids all their lives.
But what about horses? I figure a ratio of about 4:1 is fairly accurate, both mentally and physically, through their younger years.
A yearling would be the equivalent development of a child in pre-school, busy, running on instinct, trying to test their boundaries with no idea that anything can be 'wrong'. Everything goes in the mouth and bites and kicks are not yet  tempered by social graces.
A two-year old would be about eight in human years, learning social skills and fitting in with the herd while taking in some meaningful life lessons; a three-year old would be 12, tall, gangly and shy, still hiding behind their hair.
It gets more interesting as they get into their teens, with a four-year old acting like a 16-year old child, cocky and sure but still insecure, and a five-year old coming in at the young adult status of 20, ready to perform adult tasks but without the experience and confidence that time will bring. As they move through their sixes and sevens, they fill out, gain confidence, and finish growing. By eight they have blossomed into full adulthood, carrying their full size, weight and opinions. 
i remember a person with a riding school once telling me they only used horses that were 'double-digit' in age and it makes perfect sense. By the time they're 10 they have shown patterns of behaviour, hopefully received a good education, demonstrated their abilities and have proven their reliability and soundness, just like a forty-year old person.
I don't know if this formula applies as they get older. I'll have to think more about that. The average dog is regarded as senior at seven and lives until 12, with many breed differences there. There are certain breeds of horses that mature physically at different times, and I'm not sure about ponies and donkeys, who seem to live forever. The oldest mare I've foaled out was 26, Armbro Flight: she was winning races the year before I was born. The oldest horse I've known belonged to a friend in Rockwood, a Quarter Horse mare named Bordolino who lived to 48 years of age.
But why does the age ratio matter?
It might matter when you're shopping for a first horse for a child or new horse owner, looking for the seasoned reliability of a good ten-year old. A lot of people buy a young horse but ask yourself, are you prepared to adopt a teenager? Or it might matter when you're deciding how fast and hard to push your horse's training.
It matters to me so that I can reference their behaviour and my expectations of them. I want to encourage good social habits when they're foals and yearlings, like how to share your toys, respect your elders and how to act in company. I'm not a fan of riding two-year olds. To me it is like asking an 8-year old child to carry a heavy backpack and expecting them to stay sound into adulthood. They do need an appropriate education, but physically they are way too underdeveloped to carry a load or pull a cart.
I wouldn't expect a typical three- or four- year old to be confident enough to go out on the trail alone. They are still shedding teeth until they're nearly five years old, sometimes resulting in bitting issues that they may carry for a long time after the tooth fairy has come and gone. Often when they're three and four their butts are higher than their withers, putting a lot of stress on a front end when they haven't yet learned how to carry a load properly.
In terms of mental capacity, it helps me to understand their needs, like the need to have company, and my needs: the need for them to have company that is a good influence. By that I mean that it's easier to deal with a young colt who's been turned out with a wise elder than it is to deal with a couple of young punks.
But most of all, it matters so that I can help to develop a horse that is sound of mind and body who can enjoy a long and productive career.