It is difficult to predict what the breasts will look like at that age from where you are now. Overtime, most likely the breasts will sag, they may thin out, etc..
Your breasts will age as will the rest of your body over the next 60 years
It is difficult for anyone to tell you exactly what you will look like 60 years from now. There are many factors that will help determine how your breasts will mature over time, whether you have implants or not. Some of these factors can include: genetics, smoking status, sun exposure history, weight gain/loss, pregnancy/lactation, size of implant, position of implant etc.
Implanted breasts are generally age appropriate
In the best scenario, augmented breasts will drop, soften and sag in an age appropriate manner. So conditions such as pregnancy, lactation, weight gain, loss, excessive sun exposure may be expected to test the elasticity of the skin brassiere. By contrast, problems such as chest trauma, mastitis which predispose to capsular contracture, may produce firming, narrowing and distortion of the breast.
It's important to realize that cosmetic surgery operations do not stop the aging process, merely alter it, hopefully in a favorable manner. Don't worry, your breasts will age appropiately.
Results of breast augmentation over time
Unfortunately, over time, the results of breast augmentation will change. Gravity, aging, pregnancy, weight loss/gain, genetics are all possibilities that can affect your results. As plastic surgeons, we cannot guarantee lifetime results.
Breast appearance and aging
There are so many variables that may affect the appearance of your breasts -over time- whether you have implants or not. These factors include pregnancy, breast-feeding, frequency of weight gain/loss, smoking, and your unique elasticity and quality of the breast skin and soft tissue. In general the larger the natural breast and/ or the larger the implant the greater the chance of stretch of the skin over time.
At the age of 20 you can expect that you will need another surgery, at some point in your life, to replace the devices.
VIDEO (click here for link) Looking into the breast implant crystal ball
It is impossible to predict the shape and appearance of your breasts 50-60 years from now. You could look fine or develop a "snoopy deformity" or "rock in the sock" depending on several factors.