The average age a male dog begins to sexually mature is 6 months, but sexual maturity is greatly affected by the breed of the dog. Smaller dogs reach sexual maturity faster than larger breeds. Male dogs reach full sexual maturity by 12 to 15 months of age.
Male dogs can mate at any time, and they can continue to produce offspring well into old age. Female dogs also become sexually mature around 6 months of age, but female dogs only accept mates when they are "in heat," which happens approximately once every 6 months and when the female dog is fertile.