Q. At what age do dogs become sexually mature?
A. Male puppies will display mounting behavior at a very young age (3-4 weeks), but are not producing sperm. This puppy mounting behavior will often disappear in a short time and then reappear as the dog approaches puberty. Half of male dogs will display mounting behavior by 5½ months of age, and will become able to sire a litter around that age. Note that some male dogs may be able to sire a litter before 5 months of age - 5 months is the average.
Female dogs become sexually mature (have their first heat) between 5 and 12 months of age. Small breed dogs tend to have their first heat around 5-6 months, while some giant breed dogs will not start to cycle until they are 18-24 months of age.
Although most dogs are sexually mature by one year of age, dogs should not be used for breeding until they are at least 2 years of age.