When we first obtained him he was 12 weeks old, and was eating some dry food, but still received one syringe feeding per day. He loves his feedings and now goes down to the floor of his cage and begs constantly for his syringe. He calls and whines until we feed him. He still eats a little of his pellets and a little of a seed and dried fruit mix, but not much. He seems healthy and the vet can't find anything wrong with him.
Cockatoos can be very difficult to wean, as can cockatiels and a few other species.
Most of my comments here are general enough to apply to weaning difficult babies of almost any type. The first and most important concern is to make sure that the baby is really healthy. If anything suggests an abnormality, the veterinarian may suggest some diagnostic work. Assuming all seems well, then the behavioral aspects of a difficult weaning can be properly addressed. One of the best strategies is to get away from a syringe, and change over to a small spoon.