Complex sentence: At twenty years old, lured by the discovery of gold in the Sacramento Valley, with three friends, he joined the army of "forty niners" and spent the next two years prospecting for gold in California.
Split into simpler sentences: At twenty years old, lured by the discovery of gold in the Sacramento Valley, he and three friends joined the army of "Forty-Niners". He spent the next two years prospecting for gold in California.