What Causes Forgetfulness And Lack Of Concentration During Old Age?

I have a friend who is 76 years old. over the years he has been forgetting names, places, events, dates and in conversation forgetting the words to complete a sentence(I know most of the words and complete sentence or say the word needed to keep him from frustration. While driving he has started not noticing light changes from green to red. in the middle of block light is green before he drives to end of corner light changes to red and he continues driving unless I say light is red. he only remembers seeing green light. He cannot walk a treadmill and watch TV at same time, he stops walking on treadmill and falls off because he forgets he is walking and concentrates on on tv..he focuses in on one thing at a time, pays attention to detail, but cannot remember at times.