The Age of a Horse and Teeth
At best, when the date of birth isn’t known and a horse is over about 5, aging a horse from dental information is a good guess.
We aren’t usually privy to registration or birth dates of the horses that arrive into the facility.
We use the knowledge of teeth we have to assign a good theory as to the age of each horse without a know birthday.
You can look at the galvayne’s groove, cups, head x rays, how many teeth are present and projection of the teeth/mouth to help come up with an age.
But know that genetics, environmental factors, neglect and diet can alter all of those things in unexpected ways, and that of you ask 10 knowledge horse people (including vets) to age a horse, you often will get 19 different answers, often with a decade or more gap between highest and lowest guess.
We’ve many times has scenarios like this:
I age a horse as 12
Vet #1 ages horse as 22
Vet #2 ages horse as 10
Trainer ages horse as 16
Vet #3 ages horse as 12
and who knows if any of us are right?
I personally like to look often at the teeth of horses with a confirmed age and consider what I’d guess they are, if it was unknown. Doing this does improve my ability to guess more accurately, but it also often shows me that teeth don’t tell the story very well sometimes.
I have personally had a good record of accurate aging from teeth around 75% of the time when a registration or records to confirm birth surfaced, but 25% of that time I (and experts involved) gave been as much as 15 years off.
It happens.
I haven’t known a horse to be aged over 25 to end up notably younger when records have surfaced. I have known horses to be aged as 20 and end up actually only 12.
Keep this in mind if you buy a horse assumed to be on age and then aged another way later on.
Some reading on aging above 
