I've had Pip for 4 years and he was thought to be about three years old when I got him, so he's something like 7 now and still has sort of mottled tail feathers. It's not full blown barring like you see on a hen, but I've seen juvenile tail feathers that look like this.

When adult males and females have different plumage it's called dimorphism, and the type of pied that cockatiels have is called anti-dimorphic pied because it messes up the usual color differences between the sexes. It usually messes up the face color permanently, and other juvenile markings tend to stay around a lot longer than usual.

Rocko is a year old.