Ok, I'm just going to tell you my experience w/ my horse who has arthritis and you can take what you need out of it
I bought Odie as a 3 yr old as an all-around horse. We showed strictly all-around for 2 years and then switched to barrels when I realized that he was NEVER going to shorten his stride for WP. Because I was serious about running, and he was having diffuculty turning to the left, I started having a chiro come out. His pelvis was "locked", she would "unlock" it, but it never got better. So we took him in for x-rays. The x-rays showed that his left femur is a few cm shorter than his right, BUT his illium is cm longer on the left than the right. Basically, his legs are the same length, but the joints aren't even so they break at different points. Because this is how he was born, arthritis had started to develop.
The diagnosis ended his barrel career, because his butt would never be able to handle the stresses of racing. However, he is perfectly able to continue as a huntseat horse.
He's almost 22 now and has NEVER EVER taken a lame step due to the arthritis outside of a flexion test (then he will take 2 missteps, and then he's fine). Now that he's older, he does take awhile longer to warm up and get all the stiffness out, but he's 110% rideable.
I know some on here list a large amount of meds, treatments, etc for their arthritic horses. And I've worked with horses like that as well, BUT as maintenance for Odie's arthritis I pay a grand total of.....wait for it....wait for it...$36 on a senior supplement w/ xtra arthritis stuff in it. Next summer it will go up a little because I'm going to start having a chiro come out so my daughter can ride/show.
So in a nutshell, in my experience
even a young horse can get arthritis, but still be considered sound (if I hadn't gotten x-rays, and just stayed w/ all-around I still wouldn't know he has arthritis). Definately get the x-rays, BUT even if they show some arthritis, ask the vet about the prognosis. Having arthritis isn't always a death sentence.
Just my 2 cents on the topic