If you are thinking about breeding:
Boys can start to have sperm at 7 months old, but 10 months, is more common. Young dogs do not have quality sperm. Almost all dogs will have sperm by one year.
Stud dogs are at their prime between 18 months and 4 years old (or 5), after that, the quality of the sperm starts to lessen, and the ability to penetrate eggs, lessens. By 10 years old, most stud dogs do not produce sperm that will impregnate a female dog.