Ferrets reach sexual maturity between 4 and 8 months of age. Sexual maturity occurs in the first spring after birth. The vast majority of ferrets are spayed or neutered before 6 weeks of age. This is primarily because females come into heat when they reach sexual maturity and will remain in heat unless bred.
Male ferrets that have not been neutered have a strong, musky smell when they come into season, which happens twice yearly. They also mark territory by urinating on it.
If your ferret is not spayed or neutered when you get it, you should talk to your veterinarian about performing the procedure. All pet ferrets should be spayed or neutered at an appropriate age.