A mutation doesn't have to be expressed right away--it may be adult-onset. This means the mutation is not expressed until later on. For example, Alzheimer's diseasae, Schizophrenia, Huntington's disease, all show up later in life. Certain cancers such as prostate, colon and breast cancer may develop later on. People can be born with genes that increase their chances of developing certain types of cancer. They don't get the disease until mutations in specific genes occur in their life.
In Alzheimer's disease, a cause of dementia in the elderly, there are amyloid plaques in the patient's brain. A variation in the APOE gene which is involved in the metabolism of fat may play a role in the formation of these amyloid plaques. In Alzheimer's patients, excess lipids are not broken down. Other autosomal dominant mutations in APP, PS1 and PS2 genes play a role in this disease which tends to manifest before age 60.
In schizophrenia, multiple loci have been identified as to contributing to the late onset of schizophrenia in patients. In Huntington's disease which is autosomal dominant, the first symptoms manifest in mid-life. It is a neurodengenerative disorder. There is a severe loss of neuronal tissue in the part of the brain responsible for thinking and movement.
A mutation in the HTT gene, which encodes a protein called huntingtin is the problem. Each normal huntingtin molecule should have been 6 and 35 glutamine amino acids(CAG) in the HTT gene, but people with the disorder can have between 36 and 180 CAG repeats. This mutated gene represses the expression of BDNF gene. This in turn, makes a reduced level of BDNF an important protein used in the adult central nervous system. Over time, depression and cognitive impairment occurs gradually, until the disease phenotype presents in middle age.
Although someone inherits a mutated gene, it may not be expressed right away due to their environment. For example, it has been shown that exercise and diet (part of the environment) lead to production of BDNF, which significantly slows the progression of Huntington's disease.