premise: Amyotrophic lateral sclerosis starts earlier in genetic cases.
hypothesis: Amyotrophic lateral sclerosis . About 5 -- 10 % of cases are inherited from a person 's parents . About half of these genetic cases are due to one of two specific genes . The disease usually starts around the age of 60 and in inherited cases around the age of 50 . A motor neuron disease ( MND ) is any of several neurological disorders that selectively affect motor neurons , the cells that control voluntary muscles of the body .