That is nearly an impossible question to answer. There are so many factors that enter into it, such as breed and discipline.
A Quarter Horse that is a cutter, or reiner, would be worth more as a 3 year old, same as a race horse (any breed). A trail horse can be any age from 4 - 20+ (my personal preference is a 10 - 15 year old). A dressage horse at the higher levels can be in his teens - same as an event horse.
Did you have a particular reason for the question or is it something in general?
It all depends on the individual horse, his comformation, soundness, bloodlines, breed, temperament, past worklife....On average, I work a horse most between ages 5 and 18. My Paso Fino is spirited, so he's 23 and acts like a 3-yr old, but to preserve his longevity I don't work him as much as my other guys anymore.
Well I guess every horse is an individual, and I agree with what the other people have said, but an alternative view is always like most horse books tell me that horses are at their peak performance age between ages 7 and 12. Which I suppose is like being about 25 human years, no longer growing but not yet ageing.