In the wilderness areas in the 1960's my studies showed that the adults first quit bringing food to the nest about the 12th week. After 2-4 days most of the young then fledged -- often flying off to land on the beach where the adults could be seen eating. After 5-10 days of this the adults simply left - and 3-5 days later the chicks left.
I almost never saw a wilderness eagle bring food to the nest once the chick(s) had fledged. My suspicion is that in wilderness areas, almost all beach areas and perch trees are considered safe landing and feeding areas. In the more urban areas, where surrounding roads, lawns, sidewalks or even beaches are frequented by people and dogs or cars, the nest site becomes singularly more secure and gets used for the continued feeding of the young, even after first fledging.
Also in the wilderness areas, the common number of young per nest is ONE -- in the urban areas it is TWO! I already see a general trend here. In the urban areas where there is a tendency to feed the young in the 'secure' nest, this does not offer incentive for the second young to leave. The general tendency of the adults to bring food to the nest could hold both the first and second chick in the nest longer, increasing the time in the nest before fledging.
What will be the lasting impact on the survivability of the young once they leave the nest territory is a long way off being answered. Does staying around the nest and nest territory longer give advantage or disadvantage to the young? A good thesis topic!