This is very common question. First of all women undergo breast augmentation from 18 years old to well into their 60s and above! What time is best is really a personal decision, but there are several factors to consider. Undergoing a breast augmentation will not keep you from being able to breast-feeding. It will also not keep your breasts from aging or changing with time as they would do naturally.
The two major age groups that the majority of women undergo breast augmentation is at a young age (twenties or thirties) and then again in their mid to late forties (after their children have gone away to school). Patients who are much more mature in age are coming to the office more and more frequently as people are living much more active lives later in life.
No question that pregnancy, breast feeding and weight changes can all affect the breast, however many women want to enjoy their breasts at your age and if they do have changes after having children they can have other procedures once that is complete to restore their breasts to a more youthful appearance.
I encourage you to see a board certified plastic surgeon to discuss your options!