In some male dogs, the signs of sexual maturity appear earlier than others. A male dog can impregnate a female when he is as young as five months old. Some signs of sexual maturity you can notice include humping, urine marking, and over-protectiveness. Some of these behaviors can start early and intensify as he continues to mature into 12 months, or even longer, depending on the size of the breed.  Small dogs reach puberty earlier and can often have the operation done earlier. Larger breeds may need to wait longer to develop properly before neutering. The recommended age for neutering a male dog is around six to nine months.