This is called juvenile HD, I think. From studies, it seems that the younger the symptoms begin, the more severe the disease is, and the shorter the life expectancy of the patient. This younger form of HD is usually passed down from a father, but can be passed by a mother. The genetic test can be done at any age and still be accurate since the gene is there from birth. However, they will not do the HD genetic test on minors unless they are suffering from the symptoms of the disease.
People vary in terms of which symptoms manifest first.
Usually, it is very slight psychological symptoms, followed by physical symptoms. People usually notice the physical symptoms more quickly, and then in retrospect, realize that they were suffering from some of the psychological ones. Moodiness, irratibility, and depression are some of the earliest psychological symptoms. However, many many people deal with those, even if they don't have any relation to HD, so they are not definative symptoms. The earliest physical symptoms are usually slight tremors, or twitching in the hands, feet, limbs, especially when relaxed.