Most genealogists started genealogy after the older people in the family had died, so we have to find information in the public records, which are plentiful.
Census records give a lot of bang for the buck, so to speak. The 1900 U.S. Federal Census, for example, provides a month and year of birth, a year of immigration (That immigration year is also found in the censuses for 1910-1920-1930)how many years a couple has been married, and much more. We can't be sure of the complete accuracy of dates in those censuses but they give us a starting point. Death certificates often provide a date of birth. Social Security applications provide a date of birth and a place of birth and are usually accurate since we know the person filled out their own application. Marriage applications often provide an age, sometimes a place of birth and sometimes names of parents. Church records often provide a date and place of birth. There are plenty of other sources. Look for ideas of where to gather clues on sites such as these and also in genealogical books and magazines.