Sleepwalking (also called noctambulism or somnambulism), under the larger category of parasomnias, is a sleep disorder where the sufferer engages in activities that are normally associated with wakefulness while asleep or in a sleeplike state.
Sleepwalking can affect people of any age. It generally occurs when an individual awakes suddenly from Slow Wave Sleep (SWS or sometimes referred to as "deep sleep"), causing the sleepwalking episode.[citation needed] In children and young adults, up to 25% of the night is spent in SWS.[citation needed] However this decreases as the person ages until none can be measured in the geriatric individual.[citation needed] For this reason, children and young adults (or anyone else with a high amount of SWS), are more likely to be woken up and, for the same reasons, they are witnessed to have many more episodes than the older individuals

Info Source, Links, and more info:
http://en.wikipedia.org/wiki/Sleepwalking