Since you hadn't posted since March 2007, I won't go further into this:
The census of 1900 was enumerated on the 16th and 18th day of January.
Frank D. Sexton, born in Pennsylvania to father born in New Hampshire, mother in New York.
Katie, wife, December 1870 in Washington, 29 years of age, married 9 years
Mary F, Oct 1891
Mary M. born April 1894
Mary E, March 1895
John D, December 1896.
The World War I Draft Registration Card images shows William McKinley Sexton born on 7 January 1900.But, he is not listed with this family in the 1900 Census.
Was he just omitted?In the Census, Katie, wife of Frank is shown as mother of 5, but four living.In the next residence down, living as a boarder in the Jessee Scherer Home (You had also inquired about this in another search), is a Frank E. Sexton, born June 1881 (age 18).Below him is Violet E. Sexton, born June 1887, age 12, both born in Pennsylvania.
The 1910 Census for Union, Lewis County, WA
Frank D, age 59, a widower, born in PA
Francis, daughter age 18 (Mary F on the 1900?)
Mable M, age 16 (Mary M on the 1900?)
John B/D?, age 13
William, son age 9, born in Washington (age 9, born 1899/1900)The census was enumerated on April 18, 1910.So this William was born sometime between April 19, 1900 and April 17, 1901.
The WWI Draft Registration Card images show William McKinley Sexton as being born 7 January 1900.
The 1920 Census for Friday Harbor, San Juan Co
Frank D?A? Sexton, age 66, born in PA, to father born in New Hampshire, mother in New York.
Minerva E, age 76, born in Ohio
William M. son, age 18, born in Washington, to father born in PA, mother in WA. A laborer on a boat
The 1930 census for Seattle, King County
William Sexton, age 30, born in Washington, father of mixed blood, mother code "97", a mate on a tugboat.
From the enumerators instructions:
174. Columns 19 and 20. Place of birth of parents.-Enter in columns 19 and 20, respectively, the State or country in which were born the father and the mother of the person whose own birthplace was entered in column 18. In designating the birthplace of the parents, follow the same instructions as for the person himself. (See pars. 165-173.) In case, however, a person does not know the State or Territory of birth of his father (or mother), but knows that he (or she) was born in the United States, write "United States" rather than "unknown."
174a. For the Indian population, which is practically all of native parentage, these columns are to be used for a different purpose. In column 19 is to be entered, in place of the country of birth of the father, the degree of Indian blood, as, "full blood" or "mixed blood." In column 20 is to be entered, in place of the country of birth of the mother, the tribe to which the Indian belongs.
This data "indicates" a possibility that William was not a blood born son of Frank, was not born in January 1900 but later.While it "appears" that he is the son of Frank, the person in the 1920 and 1930 census might not be.