during which period did the mount erupt the first time?
The "Ape Canyon Stage".
Mount St. Helens
The early eruptive stages of Mount St. Helens are known as the "Ape Canyon Stage" (around 40,000–35,000 years ago), the "Cougar Stage" (ca. 20,000–18,000 years ago), and the "Swift Creek Stage" (roughly 13,000–8,000 years ago). The modern period, since about 2500 BCE, is called the "Spirit Lake Stage". 
who was harry r. truman? He was an American businessman, bootlegger, and prospector. when was he born? October 1896. where is he from? He lived near Mount St. Helens. where is this mount located? Skamania County, Washington, in the Pacific Northwest region of the United States. is it a volcanic mount? Yes. has it ever erupted? Yes.
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Mount St. Helens [SEP] Ancestral stages of eruptive activity
Thirty-six thousand years ago a large mudflow cascaded down the volcano; mudflows were significant forces in all of St. Helens' eruptive cycles. The Ape Canyon eruptive period ended around 35,000 years ago and was followed by 17,000 years of relative quiet. Parts of this ancestral cone were fragmented and transported by glaciers 14,000–18,000 years ago during the last glacial period of the current ice age. The second eruptive period, the Cougar Stage, started 20,000 years ago and lasted for 2,000 years. Pyroclastic flows of hot pumice and ash along with dome growth occurred during this period. Another 5,000 years of dormancy followed, only to be upset by the beginning of the Swift Creek eruptive period, typified by pyroclastic flows, dome growth and blanketing of the countryside with tephra.