Simple answers, really.
Use common sense when riding and training a horse. There are some sects of the TWH world which are rather unsavory; they put showing and competing (or just breaking) far above the horse's welfare, and the result is an unsound animal. With good breeding for soundness and longevity, a quality Walker is no different than a quality Quarter or Arab. They need proper conditioning, time to mature before hard riding, and, well, common sense. Treat them right and, barring bad luck, they'll tend to last. Ride them young, long, hard, sored, or strung out....and they won't.
If in doubt, VET CHECK!
That pretty much sums that up
My now 24 yr old TWH went on his last hard trail ride when he was 16 and the only reason I had to quit taking him on big organized rides and difficult rides is because he developed Equine Metabolic Syndrome, which is close to insulin resistance but not quite. The disease slowed him down - period.
When we lived in SoCal and Duke was 15, I spent an entire summer schooling him in the rock hills with two friends and their QH's, to go on a 30 mile roundtrip ride in October. October in the Low Desert of Socal generally finds the temps have "dropped" to around 95 - 98 degrees.
To shorten this up, my beloved 14.3H TWH (in my avatar), not only wouldn't get out of his running walk GOING but I couldn't slow him down to match up with the two QH's on the 15 miles back home. They finally told me to quit trying to hold back, that they'd never seen a horse with so much energy and who barely broke sweat. Just let him "walk on!" So I did.
Duke never sored up in any way shape or form -- that huge motor of his was in peak condition and there was no slowing him down.
Until he got sick with metabolic issues, Duke historically started large organized rides (as in 250 - 300+ horses) in the top ten and would come back to the trailer in the top ten.
My TWH that is 16.1H and really long-backed went on an all-dayer with a handful of other horses when he was 14 or 15. He was not used to going a lot but paced himself with the lead horse and never broke sweat. The guy on the lead horse tried to buy "the Energizer Bunny" from me; said that horse was "tough as nails for never getting out". Thanks but no
Then there's my 15.3H TWH whom I only bought six years ago then I had a pretty bad accident and haven't been able to ride him much. The husband of the previous owner trailed him all the time on some pretty rough climbing trails and her hubby is a big man. This horse was 12 when I bought him. He's now 16 and a lonnnnng way from being used up; it's me that can't take that kind of riding anymore
I also have always ridden bareback, so I'm sure that helps my horses tremendously.
The prices of TWH's are way down to begin with - probably more so than other breeds in this lousy economy. Just because a horse is cheap doesn't mean there's something wrong with it. What could be, is that it didn't do well in the show ring, and/or the owners have too many, are in financial straits and need to sell.
I am sorry the OP has such a negative "Old Age" perception of Tennessee Walkers. Please read Bubba13's comments a couple times to hopefully dispel those negative thoughts.
I have four horses and I am done bringing more home. If I were looking, I would never turn away from a TWH that was in it's mid-Teens provided the vetting showed the horse to be healthy.
The one thing I would caution with older Walking Horses, is to have the vetting include at least blood work to determine ACTH level and insulin level. Especially if the horse is a chunk and the Seller brags on the fact that it's an Easy Keeper.
TWH's are on the predisposed list for metabolic issues; my two chunks have them; Mr. Long and Lanky does not
Hope this helps