"the age following childhood, the age of growing" (roughly the period from the 15th to the 21st year; or age 14 to 25 in males, 12 to 21 in females), early 15c., from Old French adolescence (13c.), from Latin adolescentia/adulescentia "youth, youthful people collectively," abstract noun from adulescentem "growing, youthful" (see adolescent (n.)). Adolescency (late 14c.) is slightly earlier.
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit jara "old age," jarati "makes frail, causes to age;" Avestan zaurvan "old age;" Greek geron "old man;" Ossetic zarond "old man;" Armenian cer "old, old man."