A 20 year old could be in any number of places: At home with his parents. Married early and living with wife. Living with one of his siblings. Living in another town entirely (usually as a boarder) having moved to find work. In 1900 not that many people went to college or joined the military, but some did. And, yes, there is the possibility that he died in the interim.
Anyway, I wouldn't expect to find a 20 year old living with his parents.