The combination of old and young guinea pigs usually works fine. The older GPs are unlikely to view the youngsters as a threat, so old/young pairs should be a okay. Keep a close eye on them, though, over the first few hours, to make sure they're getting on. Check out the introducing guinea pigs section of this Guide for general tips on helping those introductions go smoothly.
Old and young guinea pigs can get on just fine
In addition to making sure the older guinea pig isn't bullying the younger one, you also need to be aware that female guinea pigs reach sexual maturity swiftly, and can become pregnant at just five weeks of age (and if they do, it's bad for their health prospects). Your young/old combinations need to bear this in mind. Same-sex set-ups can work best.