In the United States, Why Has It Become Difficult to Determine

In the United States, why has it become difficult to determine when one has become an adult?

A) It hasn't; one is an adult in the United States at age 18.
B) Children are physically reaching adulthood later than in past generations.
C) Cognitive ability defines adulthood and is difficult to measure.
D) One legally becomes an adult at different ages in the United States depending upon whether you're voting, driving, or getting married.