One big one is dehydration.

Drinking becomes inconvenient or undesirable. The result is a health problem.

Proper nutrition is another. Adequate dietary intake of essential nutrients can be a problem.

Most of the problems start when they are younger though. It takes years for your bones to develop problems. They will not get better all of a sudden either.

Much of what needs to be watched for needs to start in middle age.

There is a new term called aging management that is emerging from this. You can find a lot on anti-aging otherwise.

blood pressure, weight, the rate of weight gain/loss, visual acuity, hearing, abnormal movements like tremors, height reduction maybe osteoporosis, heart rate, respiratory, memory loss…

seriously - a good honest doctor and reliable loved ones.

otherwise, cholesterol levels, sugar levels, BP, weight, nutritious foods, mild exercise as in walking or gardening to keep joints smooth and smiles.