school-age child vs younker

school-age child and younker both are nouns.

Word	Noun	Adjective	Verb	Adverb
school-age child	Yes	No	No	No
younker	Yes	No	No	No

As nouns, younker is a hypernym of school-age child; that is, younker is a word with a broader meaning than school-age child:

school-age child: a young person attending school (up through senior high school)
younker: a young person (especially a young man or boy)

Other hypernyms of school-age child include spring chicken, young person, youth.

school-age child (noun)	younker (noun)
a young person attending school (up through senior high school)	a young person (especially a young man or boy)