Most people I know say that every horse that is anything more than a pasture pal should be on a joint supplement and its can never hurt so I say why not? I have three horses, own a horse farm with boarded horses (plus mine), and work with many barns and horses elsewhere. All three of mine are on a joint supplement. I do adjust the level of joint supplement based on the individual circumstances for each horse. It also depends on the brand/type. If you want to feed a cheap knock-off I would suggest a higher level of it. If you feed a good quality one, you can probably get a lower level and it will be equal to the higher level of the cheap one. My older horse is on a senior formula, my 14 year old heavily ridden/competed warmblood is on a much higher support formula, and my 2013 colt is on a one designed for growing, very lightly worked horses. It sounds like you have a hardworking young horse, headed for an intense career (I Event my warmblood as well), so I would say absolutely go for the joint supp. It can help prevent later issues as well. Better to stop stuff before it starts.
Good luck! Hope this helps!
The triple threat of riding = EVENTING! 😉