The rate of disease progression and the age of onset vary from person to person. Adult-onset or classic HD, with its disabling, uncontrolled movements, most often begins during middle age. There are, however, other variations of HD distinguished not just by age of onset but by a distinct array of symptoms. For example, some persons develop the disease as adults, but without chorea. They may appear rigid and move very little, or not at all, a condition called akinesia. These individuals are said to have akinetic-rigid HD or the Westphal variant of HD.

Some individuals develop symptoms of HD when they are very young--before age 20. The terms early-onset HD or juvenile HD are often used to describe HD that appears in a young person. A common sign of HD in a younger individual is a rapid decline in school performance. Symptoms can also include subtle changes in handwriting and slight problems with movement, such as slowness, rigidity, tremor, and rapid muscular twitching, called myoclonus. Several of these symptoms are similar to those seen in Parkinson's disease, and they differ from the chorea seen in individuals who develop the disease as adults. People with juvenile HD may also have seizures and mental disabilities. As mentioned previously, the earlier the onset of HD, the faster the disease seems to progress. The disease progresses most rapidly in individuals with juvenile or early-onset HD, and death often follows within 10 years.

It appears that individuals with juvenile HD have usually inherited the disease from their fathers. These individuals also tend to have the largest number of CAG repeats. Scientists believe that the reason for this may be found in the process of sperm production. Unlike eggs, sperm are produced in the millions. Because DNA is copied millions of times during this process, scientists theorize that there is an increased possibility for genetic mistakes to occur. To verify that there was a link between the number of CAG repeats in the HD gene and the age of onset of the disease, scientists studied a young boy who developed HD at the age of two, one of the youngest and most severe cases ever recorded. They found that he had the largest number of CAG repeats of anyone they had studied so far--nearly 100. The boy's case was central to the identification of the HD gene and at the same time helped confirm that juvenile patients with HD have the longest segments of CAG repeats, the only proven correlation between repeat length and age at onset.

A few individuals develop HD after age 55. Diagnosis in these persons can be very difficult. The symptoms of HD may be masked by other health problems, or the person may not display the severity of symptoms seen in individuals with an earlier onset of HD. These individuals may also show signs of depression rather than anger or irritability, or they may retain sharp control over their intellectual functions, such as memory, reasoning, and problem-solving.

There is also a related complex called senile chorea. Some elderly individuals display the symptoms of HD, especially choreic movements, but have a normal gene and lack a family history of the disorder. Some scientists believe that a different gene mutation may account for this small number of cases. Others, however, believe senile chorea is a late-onset form of HD.