Fantastic question -
But also one that is very difficult to answer.
The shape of your breasts as you age depends largely on the elasticity of the skin and the strength of the Cooper's ligaments which are suspensory structures within the breast tissue. These are the two forces at work to keep your breasts from sagging.
Larger breasts place more force on these structures and are more likely to cause sagging. The larger the implant, the larger the breast and the larger the sag.
Things that can also change the shape include capsular contracture (the incidence increases over time) and weight change. As we age weight distribution can change from the pattern when we were younger and can cause change in breast shape.
I hope this helps.