The Formal Operations Stage is the time for developing many abstract concepts.
As it was in the Pre-Operational Stage, adult-child communication is the key to rapid speech and language development, although the focus is less on structure and more on semantics.
Formal Operational Stage (11 years to the sky is the limit):
One of the skills that humans become awesome at is manipulating the world in their minds, without any actual touching turning or moving of worldly objects. This is not an inborn ability, but must be developed over time. The leap from Concrete to Formal Operations is basically that of moving from thought processes that depend upon physical manipulation to those that are free of it.
Now concepts that cannot be easily identified by pointing to something or by manipulating something can readily be developed.