Which genetic disorder typically does not appear until a person is in their late 30s or early 40s?

Huntington's disease is a genetic disorder caused by a mutation in the HTT gene, and it is characterized by the progressive degeneration of nerve cells in the brain. One of the key features of Huntington's disease is that symptoms generally do not manifest until adulthood, typically appearing in a person's late 30s or early 40s. This late onset is due to the nature of the disease, where the accumulation of the abnormal protein in the brain leads to the gradual deterioration of neurological function over time.

In contrast, Down syndrome is a chromosomal abnormality present from birth, resulting from an extra chromosome 21, and manifests with recognizable physical and developmental characteristics early in life. Hemophilia, a bleeding disorder, is also often diagnosed in childhood, particularly when patients experience unusual bleeding or bruising, related to a deficiency in blood clotting factors. Cystic fibrosis, a genetic disorder affecting the lungs and digestive system, typically presents in infancy or early childhood, with symptoms such as persistent lung infections and difficulty with nutrient absorption.

Thus, Huntington's disease stands out as the genetic disorder that typically has a late onset compared to these other conditions.