Most of the questions I get about finding an elusive ancestor are focused on the individual and some fact about an event in his or her life. One of the most common questions involves finding a "birth date" for someone born in the 1800s. Almost always "finding the birth date" is viewed as the key for identifying the individual's parents. I feel this whole line of reasoning is a misconception. It is not uncommon for the exact date of birth to go unrecorded and searching for a date is an exercise in misdirection. It is true that there may be a document, such as a Bible record or church record, that records either the date of birth or christening, but then again, the date may never have been recorded.
Approximating age and thereby estimating the year of birth may be the best that can be accomplished. Many events in a person's life are associated with their age, for example, military service, school attendance, confirmation in a church, marriage, and death. But one of the most common ways of identifying an approximate birth date is through the birth order of the children.
It is always my suggestion to focus on the family, not on the individual, to find an elusive date or to extend the family line. Let me show how I would do this by using a hypothetical situation.
Jones is doing research on his family line. He knows his father's name and birth date, his grandfather's name and an approximate date of birth, but cannot identify his great-grandfather. (You could change the facts here to push this back a generation or so, if it would be easier to understand). Where does he start? By looking for documents that might identify his great-grandfather? No. By looking for more information on his own father and his grandfather's family. The researcher needs a whole lot more information about his immediate family before he attempts to jump back two generations. I find too many researchers who know little or nothing about their own aunts and uncles and who expect to find information about their grandparents. In one instance, one researcher I was working with, could not find his own father. After a year of looking, he talked to one of his aunts and found that he had the wrong name!
Before you launch off into the past, you need a firm foundation in the present. You may need all of your own birth, school, military, marriage, and other records so you can have multiple places to look for records on your own parents. You then do the same thing with your parents' family and so forth.
But always focus on the family, the entire family.