For myself I follow the rule that someone who hasn't reached the age of seventeen in the year of release counts as juvenile. E.g. Molly Ringwald was born in 1968, so I nominated her for juvenile in 1984, but in 1985 she will have to compete with the grown-ups. In 1985 anyone born in 1969 or later counts as juvenile for me, but that's just a rule I invented for myself to draw the line somewhere.
Don't forget that a juvenile can also be nominated in the main category.