Chiilaa, thanks for posting the link! It confirms why we wait. While I don't have access to classified, scholarly scientific data such as you've presented, what you've posted corresponds with link I posted on dogs and the poor Russian eunuchs.
Again, it's a matter of personal preference. We have solid evidence for waiting. I didn't have to research to find this out though, it's based on experience of owning both types of horses.
Here's is some more text taken from Chiilaa's scholarly scientific data she posted above: When To Castrate?
Horses can be castrated at any age. I have castrated foals at one day of age and stallions into their 20s. Although both of these extreme situations were emergencies where the horses had developed scrotal hernias (the small intestine had herniated or protruded into the scrotal sac), all recovered well. Most veterinarians will agree that castrating horses at a young age (less than one year old) is ideal.
Male horses at that age have smaller testicles that are easier to remove and have less of a chance of severe bleeding post-operatively. Many people castrate horses when they become a management problem--around two to three years of age. This could be due to a desire for the horses to develop a more masculine appearance (thicker neck, heavier build).
If a horse is gelded earlier in life, then it will grow taller--closing of the growth plates in the legs is delayed with early castration. The ideal time to castrate a horse is variable and will depend on several factors, including the management of your farm, the climate, training schedules, and so on.
For example, you might want to wait to castrate your yearling colt until he is two years old. However, if the barn where you board your horse only has one paddock, turning your intact colt out with other horses invariably results in a fight when the colt begins mounting and herding the other horses. To save the peace (and injuries to your colt), it might be best to castrate him earlier.
Another example could be that you want to castrate your 2-year-old colt in January so that he is acting like a gelding by the first show in March. However, you live in central New York, where heavy snow and extremely cold temperatures are the norm in January. It might be best to wait for warmer
Weather so that regular exercise following castration won't be impeded. Or, for a little more money, you can have him castrated at a clinic where the incisions are closed and there is no need to worry about the castration sites becoming swollen or infected. What's the difference between field and clinic
And what do you know?! Scientific dog spay/neuter studies back up, and are consistent with the Chiilaa's scientific proof about growth plates in legs. http://www.alabamacaninecoalition.or...iderations.pdf