First.You have posted a limited amount of queries on this system, you may have posted others on Rootsweb or other sites.But, I shall "guess" that you are new to genealogy research.Based on that guess, let me suggest:When you post, post all the basic information that you have on the person(s) you seek.If you have their Date of Birth, that is GREAT.If not, identify their parents, siblings, or whatever you know about the family to further help find them.If you know the DOB of their spouse, etc, that helps.As noted below, I researched Nettie in the hopes of finding clues to find Annie.Since Nettie married Solomon, his information could have lead to "Jack".I NEVER assume "they could have died", until the facts identify that they did.They "could have been snatched by aliens in a flying saucer".Genealogy is about FACTS.I liken it to a criminal investigation.You identify facts, you find clues, you connect the clues to your suspect.ANY little fact could be a clue.For example, Solomon was born in Oregon to both parents born in Missouri.A William also had the same.So, it is possible, NOT definite, that this Jack also had parents born in Missouri.
The Oregon State Archives:
Shows a Nettie Yarber marriage, actually three with a woman of that name to Riley Russell, and to Clifton Cooper (One of the Clifton Cooper records could be a license date and then the marriage date).A David Solomon Yarber has a divorce record in 1923 in Grant County, "Possibly" the same as born in 1901.
Per family tree owned by MikePotts44 Solomon Yarber born 1867, died 1902, spouse Nettie Kimball born 1884.Daughter was Dora Jane Yarber.Another tree identifies Dora married to Alfred John Lee Bliss.Nettie died 1924 in Baker City, Baker Co, OR.The second family tree identified a son, David Yarber, born 1902.
The 1900 Census for Bully, Malheur County, Oregon
Solomen Yarbor (sic), employee, born Nov 1866 in Oregon to both parents born in Missouri
Nettie Yarbor, boarder, born Jan 1885 in Oregon to both parents born in Arkansas.
William H. Yarbor, born Jan 1875 in Oregon to both parents born in Missouri.
The 1910 Census for Mountain, Wheeler County, Oregon
Clifford Cooper, age 43, born in Michigan, second marriage
Nettie Cooper, age 27, born in Oregon, third marriage, married 0 years, mother of 2, 2 living.
David Yarber, stepson, age 9, born in Oregon, to both parents born in Oregon.
Dora J. Yarber, age 7, born in Oregon
Otto Cooper, age 17.
--The two children named Yarber, could be her own children versus the children of her sister.
The Western States Marriage Index
shows Jack Yarber married Annie Kimball in Grant County 23 May 1898 at the home of C. Ladrow.
--The 1900 Census had no Ladrow in Oregon, only a few in Minnesota.There was a Charles Ladro in South Fork, Grant Co.
Now, as you indicated, they disappear.Newspapers in that time period "might" have a notice of the marriage.Obituaries were NOT common during that period of time, especially if someone did not live in an area long, or no member of the family wanted to pay for the publication.There were other factors of course.
In this case, we have someone that appears to be only 15/16 years of age into a marriage.While this occured, perhaps more often than today, the speculation of "Why" comes into picture.Again, in genealogy, speculation has no place in the investigation.But, it allows to open up the thinking process.Was she pregnant?Were they planning to move somewhere?This process only indicates you can't limit your investigation to that one area.Yes, begin there, but what additional do you know.Was he in the military?Did he enlist for service in the Spanish-American War?Keep these options open.
Now, what additional information do you have on this family that might further identify where she went?Again, a date of birth would be great.