The difference between the egocentrism of adolescents and that of preoperational children is a well-developed theory of mind in adolescence.
Egocentrism with preoperational children is pretty much unconscious - it refers to them referring to the world based only from their own perspective. On the other hand, egocentrism with adolescents has to do with higher mind processes because their minds are much more developed than that of a child.
