While we normally think of naturalization as a two step process whereby the alien first declares his intent to become a citizen and then petitions for naturalization, there were exceptions to that procedure.
For example, from 1824 to 1906, aliens who came to the U.S. while under age 18 could effectively declare their intent to become a citizen at the same time they filed their petition for naturalization once they had reached age 21 or more and had lived in the U.S. for five years (three of which as a minor). Let the law speak for itself:
So, to summarize: the alien still had to meet the five year requirement for residency, and three years of that had to be while he was a minor.
Many courts used specific forms for these cases that combined declaration of intent language and petition language in one document, and they made sure to include the word “minor.” Some may say the applicant “arrived as a minor,” while others will have the words “Minor Naturalization” emblazoned across the title or as a watermark.
For more on naturalization, see Naturalization Records and Women and Naturalization, Part I and Part II.