Melanie has traced my ancestry back to the 1500's and her ancestry even further. Many of my relatives had no birth dates and some of the tombstones in a family cemetery have only flat fieldstones with a crudely carved name for headstones. Having some idea of where they were born can help if you search US Census reports. It will at least narrow it down to a 10 year span, then if there are multiple children, you may be able to better define a birth date.
Marriage certificates and court records sometimes shed light on ages, domicile and other information that can be cross-checked with one another.
She found that her ancestry was better documented because the system in the Northern States and Europe had been well-established, whereas my relatives that were born from the Carolinas on down had very skimpy records. Those who were born in the Northern States had good records, as well as those from England, Wales, and Scotland.