I am nearly positive I have uncovered the correct baptismal record of my great grandfather, William O'Brien, in Ireland - with a name like that it's like looking for a needle in a hay stack. The year is listed as 1874 and that date is used again on his Chicago death certificate. However, most US censuses, the passenger list, his naturalization papers, and marriage certificate all list 1877.
Is there something in the immigration rules that would have made a single Irish male change his age from 19 to 16?