Dressage: longevity of the horse

To open discussions on this brand new forum, some matters of concern:

-What are the precautions trainers should adopt to get from their horses their best dressage performance for the longest time?
-At what age should the horse be started (breed dependent?)?
-What kind of progression should they follow?
-At what age could horses produce their peak performance?
-For how long can they stay near their peak?