No, a stallion is a male horse, more specifically one that can breed. (as opposed to a gelding, or castrated male, who cannot breed)
I know that they can successfully mate as young as 2 and possibly younger. Generally, though they aren't used for breeding until the age of 4.
Can a man with one testicle breed? The answer is YES
Yes. Breeding a horse to a donkey yields a "mule". Mules are sterile and cannot produce offspring.
You are really not supposed to, due to the fact that both the mare and stallion run the risk of getting injured.