Being a rider for young horses is one different profession. As a rider you may want to have an experienced horse which can be between 9-12ys or an older one for a beginner around 13-14ys to start competing on a certain level.
When it is an old horse, it can teach you or improve your confidence for a limited time. This is a good way for a beginner to start competing.
If you want a horse to help you and also build you, you may choose an experienced horse who is not so old.
But on the other hand, if you have a limited budget and if you can not afford an experienced horse, because young ones doesn‘t have enough experience they are usually cheaper, you can have a young one and you can grow together.
The good part of having a young one is, there is a possibility to have a future star in your stable.
Or it is very nice to get used to each other with a horse, as he will grow up with yout style, he will be used to your aids, strength or style more than any other rider.
On the other hand, to become better as a rider and to make your horse a future star or you need to trust your ride and knowledge, or you need have a very good trainer, either way young horse means patience.
It is extremely hard to win the trust of young horses who have been badly started by wrong trainer or people. But after finding out the solution with patience, gently you can overcome the previous problems.
Every horse has their own mind and personality even though it is a young one, it will show you the personality. For example if your horse is calm at age 4, it will be a calm horse most of the time. If you are buying your first young one, look for one who has scope and nice personality. Every horse retains his natural insticts but also they will show you their true personality.