To obtain the citizenship of the U.S, one has to be at least 18 years old.
Answer: 18 years
Explanation
Citizenship provides the rights same as that of a native in the country.
To be a citizen in the United States, one has to be at least 18 years of age and must have been obtained PR for minimum of 5 years.
In case if the individual is married to a citizen of United States of America, then it is narrowed down to 3 years.
Before 18 years, minor children have other restrictions to become a US citizen.