As the family historian you have attempted to follow the course of numerous ancestors' lives using census, birth, marriage and death records. You have when and where a certain person was born and may even be able to track them for years. Then a major roadblock, you do not find them in the family home or town, it is as if they disappeared. No record of a death, so what happened? There are many possibilities, but one that is overlooked, especially for those ancestors of the early 20th century and back into the 19th century are poorhouses.
In America when an individual or even a whole family could no longer care financially for themselves, the local county or state established institutions paid from tax monies to assist those in need. Similar terms for poorhouses were almshouses and poor farms. Here the basic necessaries were provided; such as food, clothing and shelter. A person in such need was known as a pauper.
In the beginning the poorhouses had all types of individuals; orphans, elderly, mentally ill, besides those who had fallen on hard times. In the late 19th century that was changed, with separate housing and care for orphans and the mentally ill. Even families with too many children were known to sent a couple extra children under the ages of 15 to live with other relatives, friends or even strangers. If that was not possible then those children could be placed in a poorhouse.
At any age, if a researcher is unable to locate an ancestor, it is worth checking the local poorhouses to see if they were living there. A good site online to start learning about poorhouses, those in the various states, plus Canada and a few European ones is The Poorhouse Story.
Information on facilities in all the states along with images of many of the poorhouses is provided. The site does not have a database with all names of anyone ever in an American poorhouse. Instead there is information about those institutions that existed and what is available for researching as far as documents. A few of the states have some online listing of residents at certain time periods.
This is an interesting site to investigate, even just to learn more about what may have happened to our or other people's ancestors.