A colt can be castrated at any age as long as both testicles are descended. The testicles are usually in the scrotum at birth. However, it is not unusual for one or both not to descend before the tenth month. Occasionally the testicles are not completely descended until the twelfth month. Many farms will castrate during the winter or spring months prior to the colt's first year of age. Insects are less of a problem, and cooler weather should help prevent swelling.
