More or less equivalent to "teenager", a betweener is someone of an age between child and adult. Specifically, they have had their Childhood's End Rites but not had their Adulthood Rites. They are probably physically capable of reproduction (since childhood's end rites are based approximately on menarche), but they are not considered mature enough by their society to form an oathbond. There are Asterist social and legal sanctions against sexual activity with betweeners by adults, though in some societies (especially Earthist ones) some sexual activity is sanctioned among betweeners themselves, and even adult-betweener relationships are often accepted. Sexual activity with or between children (those who have not had childhood's end rites) is universally proscribed, including in the Code of Willing.
The exact beginning and ending age varies at different places and times, but betweener status generally begins around 11-13 and ends around 16-18, lasting for approximately five years. During these years, betweeners are prepared for their adult responsibilities and roles.
For the educated classes, status as a betweener corresponds to the last years of schooling, and so high schools are often referred to as "betweener schools".