The reason for the diffenences in age DOB's for 1900 and 1910 is that each census has it's own calendar year.Each person must age either 9 or 10 yrs. from one census to the next.Each census has it's own calendar year.If a person's birthdate comes before the beginning of the census year he/she will have aged 10 years. And if the birthdate comes after the beginning of the census calendar year, then that person will only have age 9 yrs.Use the DOB of the WWI draft registration.
NH Vital Records - .Birth records prior to 1909 and death, marriage and divorce records prior to 1959 have been made available to the public for genealogical research.
Remember, some ppl were born at home and the birth was never registered.