Why does ALS start in middle age?
Short answer
The causes of ALS are not well known. Some familial types are associated with toxic proteins, susceptibility to toxins and defective protein metabolism. My personal 2-cents is that metabolic and toxicologic defects tend to cause cumulative tissue damage over time, perhaps explaining the delayed onset. 

Background
Amyotrophic lateral sclerosis (ALS), is a progressive, fatal disease characterized by the gradual degeneration and death of motor neurons. 

In 90 to 95 percent of all ALS cases, the disease is sporadic, meaning that it appears apparently at random. Individuals have no family history of ALS, and their family members are not considered to be at increased risk for developing it.

About 5 to 10 percent of all ALS cases are inherited. About one-third of all familial cases result from a defect in a gene known as “chromosome 9 open reading frame 72,” or C9orf72. The function of this gene is still unknown. Another 20 percent of familial cases result from mutations in the gene that encodes the enzyme copper-zinc superoxide dismutase 1 (SOD1). In case of mutations in the SOD1 gene it is though that mutant SOD1 protein is toxic.  

Other inherited forms are believed to change in the processing of RNA, that may lead to ALS-related motor neuron degeneration. Other gene mutations implicate defects in protein recycling. And still others point to possible defects in the structure and shape of motor neurons, as well as increased susceptibility to environmental toxins. 

Source: NIH - ALS Facts Sheet