Depending on where you lived, if you were born before the 1910s, you may not have had a birth certificate. New England required them as early as the 1640s. In the South, it was much later. Yet, lineage societies request “proof” of an individual’s birth.
An SS-5 can often be submitted instead of a birth certificate. When Social Security came into effect into the 1930s, an individual needed to apply for a card in order to be eligible. When they did so, they completed an SS-5. These applications, which catch many of working age as of 1936, often list the date and place of birth of the applicant as well as the names of their parents. As the individual is completing the form themselves or providing the information, it can be considered reliable to the extent of their knowledge. A significant number of ancestors will not be documented: an individual had to work outside the home to be eligible, and an exception to the original law meant that domestic and farm workers were not included. These exceptions were closed as time went on, but beware of them when documenting earlier ancestors.
A deceased person’s SS-5 can be requested here:https://www.ssa.gov/foia/request.html . It helps to know the Social Security number, many of which can be found using the Social Security Death Index (this index is no longer regularly updated).