DDD predominantly occurs with aging. But there are other risk factors that accelerate the degenerating process of the disc - Cigarette smoking, repetitive mechanical lifting of heavy loads, trauma and failure to do exercise. Here's a short video of a presentation that i made stating the symptoms, treatments and methods used for detecting a degenerated disc. Questions, if any, can be asked and will be answered.

References - Internet, Youtube, Books by Dr Panjabi, Dr Adams, Dr Bogduk et. al.