[nihilocrat]

Thus says the Online Etymology Dictionary (http://www.etymonline.com/):

teenage 1921, formed from -teen as a separate word + age; derived noun teenager is from 1941 (the earlier word for this was teener, attested in Anmer.Eng. from 1894). Teen-aged (adj.) is from 1952; shortened form teen is from 1951 (though this had been used as a noun to mean "teen-aged person" in 1818). Teeny-bopper is recorded from 1966, from teen but also felt as infl. by teeny. For second element, see bop.

Couldn't find the word "adolescence", unfortunately. Based on this, you could argue that the first "teenagers" showed up as early as 1894 or as late as 1941.

[whughes]

Adolescence is from antiquity, but the meaning has diluted to become the same thing as 'teenagerhood' recently, if I recall correctly. It was more like being a 'young man' prior to that.