Adult vs child. Juvenile onset means something that begins in childhood- usually before age 18, later onset suggest it begins in adulthood, .
Wording confusion. Later onset is often used to describe an affliction that comes on at a later time than a similar problem while juvenile onset describes an occurrence in childhood or adolescence. Some bacterial diseases are called early or later onset using exposure as a point of reference. For diabetes, juvenile onset applies to kids, later onset applies to adults.