If your older dog fails to recognize you anymore and is spending his nights wandering the hallways, he likely has some form of canine dementia. Scientists estimate that there are more than 30 million geriatric dogs (over the age of 7) in the United States, and more than 15 million in Europe. (source)
Dog dementia, clinically known as Cognitive dysfunction syndrome (CDS) represents a group of symptoms related to the aging of the canine brain. These changes ultimately lead to a decline of memory function and learning abilities, alteration of social interaction, impairment of normal house training, changes in sleep–wake cycle and general activity. The initial symptoms gradually worsen over time. (source)
Pacing back and forth or in circles (often turning consistently in one direction). Getting lost in familiar places. Staring into space or walls. Walking into corners or other tight spaces and staying there. Appearing lost or confused. Waiting at the “hinge” side of the door to go out. Failing to get out of the way when someone opens a door. Failing to remember routines, or starting them and getting only partway through.
Barking for no apparent reason and/or for long periods. Ceasing to bark when the dog used to be very noisy. Forgetting cues and trained behaviors she once knew. Exhibiting motor difficulties like difficulty backing up (aside from physical problems). Startling easily. Getting less enthusiastic about toys or stopping playing altogether. Performing repetitive behaviors. Having trouble with eating or drinking (finding the bowls, aiming the mouth, keeping food in mouth). Losing appetite. Failing to respond to her name. Having difficulty getting all the way into bed.
Trembling for seemingly no reason. Falling off things. Getting trapped under or behind furniture. Sleeping more during the day and less at night. Forgetting house training. Having difficulty learning anything new. Seeking attention less; getting withdrawn. Acting indifferent or frightened of people she once knew. Having trouble with stairs. Getting generally more fearful and anxious. (source)