No. Mount St Helens is a a relatively young volcano at about 40,000 years old, but by no means the youngest. That title goes to Novarupta, which first erupted in 1908. The youngest volcano in the continental U.S. is Sunset Crater, which is just under 1,000 years old.
It has several volcanoes that form the Cascade Volcanic Arc. Volcanoes in the range include Lassen Peak, Mount Shasta, Mount Mazama (Crater Lake), Mount Hood, Mount Adams, Mount St. Helens, Mount Rainier, and others