Re: Desexing Age? Sexual maturity?
Ugh. If they're together, please separate them immediately.
Males can impregnate females at about 4 weeks of age, females can get pregnant at 4-5 weeks of age. Pregnancy is very hard on sows at any age, and the death rate for sows and pups is high. But it's much higher for baby sows and their pups, and should be avoided at ALL costs.
In addition to the maternal/pup death rate, guinea pigs are susceptible to several painful/fatal genetic diseases, and they should never be bred by someone who doesn't understand the risks and the conditions that produce pigs with those diseases. In fact, many of us think they shouldn't be deliberately bred at all. There are enough accidental pregnancies to keep the species going without another pig ever being purposely bred.
Most vets require them to be at least four months old before the male is neutered, and they have to remain separate from the female for four weeks after the surgery.