If you breast feed then the advice is the same but from when you finish breast feeding.
Everyone is different so the choice must be made for each person. In practice, round, textured, silicone implants of between 260ml and 340ml give a very pleasing result for the majority of patients. However, there are a number of alternatives and options you may wish to consider. Implants may be round or shaped (anatomical or tear drop). They may be silicone or saline. Implants may be smooth or textured on the surface and they can be of differing volumes.
It is best to wait until the changes that occurred in your breasts during pregnancy have settled. Usually following birth the breasts reduce in size over several months. Once these changes have stopped then you can decide on whether you would like implants. Usually this would be 6 months after the birth. Many women put on weight during pregnancy, so it would be sensible to return to your normal weight and ensure this was stable before undergoing surgery to change the size of your breasts.
No! As long as you are in reasonable health then age is not an important consideration. I have revised (replaced) a silicone implant in an 86 year old who was delighted! And I have carried out a breast augmentation on someone in their seventies.