Katharine Hepburn's tremors in On Golden Pond were real. She developed essential tremors, a disease that can affect the extremities by causing them to involuntarily shake in varying degrees.
Per a website specializing in health-related information for older adults:
"...her tremulous voice and the persistent involuntary movement of her
head, particularly during her performance in "On Golden Pond." These are classic symptoms of essential tremor that has affected the head and voice. She never appeared to have any difficulty walking. Ms. Hepburn did not have Parkinson's Disease, and despite her "disability," she was able to perform for many, many years."
Here is an additional excerpt from a New York Times Q&A where someone asked "Why did Katharine Hepburn's head shake?":
Katharine Hepburn's ailment, recently discussed on television by her
niece Katharine Houghton, was not Parkinson's disease, but a
progressive yet treatable neurological disorder called essential
tremor. Ms. Hepburn's was referred to as familial tremor, the
inherited form. Each child of a parent with the disease has a 50
percent chance of inheriting a gene that causes it. In other cases,
there is no family history.
The disorder, which varies in location and severity, used to be called
palsy. The International Essential Tremor Foundation, which offers
information and helps search for a cure, says the disease stems from
abnormal communication among areas of the brain, including the
cerebellum, thalamus and brain stem. The group estimates that 10
million people in the United States and 200 million people worldwide
have some form of the problem, often undiagnosed.
Many do not seek treatment because they fear Parkinson's or think
tremors are part of aging. Shaking, usually starting with the hands,
can begin at any age but is more common in older people; tremors
starting with the head are more common in women.