Gregg, F. M. (1938). The age of loyalty. In F. M. Gregg, The psychology of growing personality (pp. 265-301). Lincoln, NE, : Personality Press.
http://dx.doi.org/10.1037/14700-008
The difference in the gang tendency is now marked by a spirit of greater permanence and compactness in the gang organization. On account of the more enduring devotion to the gang, clique, or club, the period has been called by Lee the "Age of Loyalty." This term we adopt because it seems to characterize so fittingly the spirit of these years. Technically it is referred to as Early Youth. The complete period of adolescence, or youth, includes the years from twelve to twenty-four, while Early Adolescence includes the first three years of that interval. The age limits are not exactly twelve to fifteen, for with girls it usually begins and ends a full year earlier than these limits, and with boys it may be even later. There is considerable variability both in the inception and in the termination of the period due to the fact that it is not age in years so much as age in physiological conditions that determines the limits. These conditions are the culmination of the rapid growth which comes between the quiescence of later childhood on the one hand and the arrival of puberty, or sex incipiency, on the other hand. (PsycINFO Database Record (c) 2016 APA, all rights reserved)