The Instructions for Enumerators of the 1900's us Census states:
Citizenship 148. Column 16. Year of immigration to the United States.—If the person is a native of the United States, leave the column blank. If he was born abroad, enter the year in which he arrived in the United States.
Column 17. Number of years in the United States.—If the person is a native of the United States, leave the column blank. If he was born abroad, enter the number of years since his arrival in the United States. Disregard all fractions of a year. If the time is less than one year, write ‘‘0.’ Endeavor to get the exact number of years in all cases.
The question of immigration (columns 16 and 17) applies to all foreign-born persons, male and female, of whatever age. It does not apply to persons born in the United States.
So, this census image below, there is some shorthand symbol being used, but it is illegible. Does anyone know for sure what this was in the St. Joseph County of Indiana?