The US county marriage records, particularly on many of the printed forms are especially useful as they have, in many cases, the exact birthdays of the bride and groom and the names of the parents of both. At least on Ancestry.com the transcription doesn't have the full birthday but it's on the image and just needs a little work to figure it out. It's usually in the style Groom was 25 years old on March 14 of [year of most recent birthday.] You do have to see when in the year the marriage was compared to the birthday, but it's straightforward to calculate. I expect the forms were done that way since all you had to ask the bride and groom were what's your birthday and how old are you?.
