means A young person, especially a child or adolescent, often spoken of with a mix of fondness and faint exasperation by someone older.

from Built from "young" plus the noun-forming ending "-ster," which once denoted someone associated with a thing or quality (as in "trickster" or "gangster"). "Young" itself is ancient, tracing back through Old English "geong" to a Proto-Germanic root and a deeper Indo-European ancestor — possibly related to Latin "iuvenis" and Sanskrit "yuvan," both meaning young. The "-ster" suffix had earlier feminine leanings in Old English ("-estre"), but by the time "youngster" appears, roughly the late 16th century, it simply tagged a person as belonging to the tribe of the young.