One criticism of the concept of emerging adulthood is that

Emerging adulthood is a proposed concept that refers to the phase during late teenage years and early twenties. These young people still are not independent from their parents and still have no children. However, one criticism of the concept is that it is exclusive only to those with a high socioeconomic status, especially those living in developed countries. It does not take into account those who have been forced into adulthood due to socioeconomic factors. Thus, it cannot be considered a distinct developmental stage. 

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.