Horse riding, as I'm sure you know, is a very long process of learning. The length of time to start training at Olympic level depends on many factors, including the work you put in and the opportunities available to you. At this stage, you are still very much a beginner. Taking your time and setting a good foundation will leave you much more open to success than rushing into it and then getting stuck at a certain level as you've acquired too many bad habits. It will be a few years yet, so learn as much and as well as you can. And remember that you never stop learning with horses.