People inherit their genes from their biological parents. At birth they have the same set of genes they are destined to live with throughout their life. The most fundamental cause of genetic variation for health is the inherited genes themselves. Family history, or the incidence of specific diseases in a family, is a good indicator of a person's risk for the same diseases as people they are related to. This is why people say some health conditions "run in the family" - there is a genetic predisposition for them. A person in middle age will benefit from knowing their family health history, and preparing for any diseases that are likely to have been inherited from family genetics.
On the other hand, an individual can inherit genes that make it less likely for them to suffer from specific diseases. Those who are genetically "blessed" in this way may find themselves with relatively long, healthy lives well into middle age. Once again, these genes are simply inherited, and cannot be changed no matter what an individual does.
There is another cause of genetic variation, however, that is a direct result of an individual's living conditions. This is through acquired mutations in genes. Genes can mutate for a variety of reasons, and at any time, but as people age there are more opportunities for the mutations to spread. Many genetic mutations are a result of environmental conditions. For example, exposure to large amounts of ultraviolet radiation, possibly from direct sunlight, can cause mutations in the cells that may later lead to skin cancer. Therefore, a middle-aged individual's health will likely be affected by the way their particular genes interact with the environment in which they live.