Background to the quote is in relation to why some young seem to take along time to moult.
Seems from this that it's possible that leaving the young in may produce unexpected results (if you'll pardon the pun)!...sexual activity may occur extremely early in this species. Young males usually start with their song as soon as they leave the nest. Courtship behaviour appears a few days later. I have seen young birds courting and copulating while still being fed by their parents....when the young are independent of the parents they may even start breeding straight away - when only about eight weeks old... I have seen several families with parents and young in juvenile plumage.