How old before a female pig can have babies?

Actually, the question should be: "How young must they be?". For their first litter, a pig must be older than two months and younger than six months to be impregnated.

Don't wait too long if you are going to breed them. When the female guinea pig doesn't have any offspring for about a year or two their pelvic bones fuse making it impossible for them to have babies. Ideally they would be between two and four months when they fall pregnant. But no two guinea pigs are exactly the same.

ANSWER:

The best time for guinea pigs to have babies are between 5-8 months old. It can be dangerous for them if they get pregnant before or after then. But it's always best NOT to breed them at all!! Because it can be just plain dangerous at any age!
These animals are rodents and can breed at a young age. They just need to be at least 4 months old. Breeding young is not advised. Wait until the animals are 6 months or more.