Guinea pigs can breed safely anytime from 4 months to a couple of years of age.  It is best for the boar (male) to be over 5 months to produce stronger babies, though it is fine for a sow (female) to become pregnant slightly before then. 

Usually any boar and sow, though sometimes a bit nervous around each other at first, will mate.  A boar and up to 5 sows can be placed together for several weeks, in which time they will mate more than once, to ensure pregnancy.  Although both parents are usually very caring toward their young, it is best for pregnant sows to have their own place to live to be safe and reduce stress.

The pregnant sow will grow very large around the mid-section in the following weeks.  After around 50-55 days you should be able to feel the babies moving inside of her by placing your fingers gently on her sides.  Handle the sow as little as possible nearing the end of her pregnancy so as not to harm the babies.

After 62-64 days the sow should give birth to 2-5 young.  It is an amazing site if you are lucky enough to witness the births.  I find cavies tend to give birth in the middle of the night when all is quiet, from about 1-4 am.  The babies are born with fur and their eyes open.  Within a few hours they can move about and even eat solid foods. 

At 4-5 weeks the babies should be weaned from their mother and placed seperately according to sex.  To sex the young(or adult guinea pigs for that matter) turn them on their backs and gently apply pressure above the genital area.  The penis will protrude in males and, of course, will not in females.  It is not easy to sex guinea pigs, especially when they are younger, so its best to have an experienced breeder check as well.