As a pet grows old, like people, their needs change. A dog that previously could hold it all day while you were at work likely can't anymore due to incontinence, dementia, or simply an old bladder that is no longer so elastic. A cat or dog may be more snappy, due to constant pain from arthritis. A cat that could previously jump up to where you had his/her water and food or pet door may no longer be able to due to degeneration of hips and knees; blown ACLs are extremely common in old cats (and recoverable without surgery as I discovered with my Raja). GI problems also present - food that may previously have been fine may now cause UTIs or vomiting and diahrea. Loss of teeth due to aging, and dehydration (they often drink less water when they get older) give rise to the need for canned food even if previously on dry - easy to chew and 80% moisture.
Some tips to deal with a pet struggling with old age: