The oldest old

The elderly who are about 85 years old often lose senses and become children again.
My grandfather is 90 years old and he can't eats by himself, my father has to help him eat and drink. Sometimes its difficult to him walking around  the house because his legs hurt and his back too. Some old people who are 90 years old can walk, eat, and do many things  by themselves because  in the past they had a heathy lifestyle, in the past, my grandfather had no a good lifestyle, he never did exercises, had no good feeding and he drank a lot of alcohol. My grandmother is 85 years old she eats healthy food and never drank alcohol, now she can walk normally ,eats by herself and cook