taking care with baby

How much is too much? Some trainers suggest not working the young horse too much, or very often, especially if a warmblood, to prevent long-term injury. Other trainers believe that if you're not at level 5 or 6 by 6 years old, you aren't progressing fast enough. My trainer noticed yesterday that delph is bum high, so is still growing-as you would expect with only being 4. And she is so amiable, and training so well, that we are moving forward. If we stayed at the same level, she would be bored. If we didn't go forward in her training, condition would be lost, as I would have to back off. So I guess I'll keep on, watching, varying the work/riding, and hope for the best?? How to know when too much is too much??