Male dogs can normally breed by 6 to 8 month of age, as they do have active, mature sperm by this age. However, if intentionally breeding, it is always best to wait until full body maturity, about 2 years of age. This is so that their pelvic muscles and hip muscles are strong enough to maintain themselves during mating without having to overexert themselves, or even "give up". Also, chances of a larger litter are better at maturity, as all of the sperm are then mature, and you'll have a better idea of the adult health and temprement of your dog for passing along genes.
Hope this helps, and let me know if I can help further.