What Makes a Horse Old?

I've read a few blog posts in the last couple months from bloggers talking about getting their horse's joints injected, needing to change warm-up routines to help with stiffness, evaluating what workload is appropriate, etc. I'm not interested in getting into the debates about what interventions should or shouldn't be done with horses, but it's had me thinking, what causes a horse to get physically old?

Note: By physically old, I mean having issues with stiffness or arthritis, or not being able to maintain fitness or good body condition, when a specific injury or illness isn't to blame.

The blog posts I mentioned are about horses who range in age from about 8 years old to 20 years old. When I was younger, I was basically taught that a horse over 20 is "old", but that maybe you should start being careful about not introducing new physical demands when a horse gets to be about 15 years old (e.g. not breeding a maiden mare after 15, not training a trail horse to jump at 18, etc.). However, there's obviously much more to it than that. You see some horses who are ridden and even compete well into their mid-20s, while there are other horses who need to retire as early teenagers.

These are the factors I can think of (in no particular order):

1. Age

How many years has it been since the horse was born?
This is the most obvious and self-explanatory. A 30 year old horse is going to generally have issues that a 3 year old doesn't.

2. Breeding

What breed is the horse? What bloodlines does the horse have?
Ponies generally live longer than draft horses, and longevity is a characteristic that can be bred for within breeds. However I don't know how much this impacts things by itself, or how much of it is tied to 3. below

3. Conformation

How big is the horse? What kind of conformation do they have?
See previous comment of ponies vs. drafties. Also, I know that if a horse has a conformational flaw, such as crookedness, it can put additional strain on the joint and result in earlier arthritis.

4. Workload

How often is the horse worked? How strenuous are the work sessions?
It make sense to me that a horse who is ridden hard 6 days a week might wear down faster than one who only does easy trail rides every couple weeks. However, considering pretty much all horses would spend more time in the stall, paddock or pasture than they do under-saddle, how much of an effect does this actually have?

5. Free time

What does the horse do in their off time? Do they amble around or do they run up and down the fence-line, playing rough with the neighbors? Do they live in a stall or a big field?
I've always learned that a "natural" horse lifestyle where the horse continuously takes small steps throughout the day is best for keeping joints healthy. But how much does this impact things overall?

6. Type of work

Does the horse jump large fences? Do sliding stops? Never go more than a walk?
I don't want to rail on other disciplines, but certain horse sports are harder on a horse's body than others. How high of a level of the sport is certainly a factor too.

7. Fitness and Nutrition

Has the horse been kept fit all their life? Are they fat or skinny? What nutrients does their diet include?
Getting into all the articles and ads about what supplements a horse needs is completely overwhelming. But how does this compare or counteract other factors? Also, I imagine an overweight horse would put stress on their joints the same way that happens with overweight humans.

8. Weight and Balance of Rider, Weight and Fit of Tack

How much weight does the horse have to carry? Is their movement inhibited by a pinching saddle or leaning rider?
I included these together, because a heavy rider isn't necessary as harmful to a horse's back as a lighter rider who bounces all over the place or an unbalanced set of heavy panniers.

9. Age of Being Started Under Saddle

How does being started too young effect a horse later in life? If a horse was never ridden until they turned 8, does that add on rideable years later?

Can you think of any other factors?

Often it might be hard to distinguish between some factors. For example, OTTBs are sometimes referred to as high-mileage, but as well as having a large work-load, racehorses mostly share similar factors of type of work, age of being started, breeding, and fitness, at least at the time they come off the track.

In my experience, I had a 20 year old anglo-arab mare who was in fantastic condition and not showing any signs of being old when I tragically lost her to colic. Other people frequently thought she was 10 or 15 years younger than she was. She never indicated any joint stiffness and looked great and felt great. However, the cause of her colic and her earlier founder were never determined despite testing, so maybe her digestive system was suffering from age in some unknown way. Ellie lived in a field as much as possible. Even when I couldn't ride much she would keep herself in fantastic shape by running around the pasture. She was started as a 4 year old, and was used to jump at first, and then as a low level pony club, 4H and dressage mount. She was ridden an average of 2-3 times a week for most of her life. (It feels weird to boil down Ellie's time with me into this little paragraph, I'll have to tell her full story on here at some point)

Ellie at 20 years young

Now, I now have a 13 year old grade mare. She wasn't started until the age of 8 and she had maybe 60 rides on her total when I got her 1.5 years ago. At the pre-purchase vet check, I was told that she looked great, and due to her low use, her joints were like a younger horse's. However, she is not super young by absolute age, and she is of unknown breeding so I don't have any information on that factor. This leaves me wondering how many years we might be able to work before she shows signs of "oldness". (I will of course keep an eye on how Kachina is feeling and looking, work with my vet, and act accordingly to her individual needs, but it's an interesting subject to think about)

Kachina when I went to try her - Age 12

The scientist in me knows that a sample number of 2 is not nearly adequate to make any conclusions. As much as I wish I could own hundreds of horses to do a more in depth report, my lottery ticket hasn't come up yet =P . So, instead, I will turn to the super scientific method of getting anecdotal information from the internet!

What factors do you think make the biggest difference in how physically old a horse is?