There is more than one theory of forgetting, based in various scientific philosophies. Biological theories of forgetting will come to different conclusions than psychological theories of forgetting.
For example, under the Trace-Decay theory, the mind cannot retain short-term memory for very long unless it is constantly repeated. This is a physical theory, based in the idea of a "trace" created in the brain in the neurons and neurochemicals that allow mental function. This means that all memories will fade with age; as the time between the trace-creation and an attempt to access that trace increases, the trace becomes more and more decayed.
However, under the Cue-Dependant theory, memories are accessed in response to external stimulus, or cues. This allows the idea that memories are never truly lost, but only "forgotten" until the cue is repeated. In this theory, memory is semi-permanent depending on how easily the mind can link the stimulus to the memory; if the mind's understanding of the cue is degraded, such as with trauma or age, the memory might be inaccessible even if is not truly lost forever.
Memories lost due to physical trauma may be lost forever, since the brain's ability to function is dependent on its physical health. If parts of the brain are destroyed, anything stored in or across those neurons can be lost and will need to be relearned. Diseases and age can degrade neuron activity to the point where memories cannot be accessed, although this does not prove that they are truly "erased."