According to piagets, the period of pre operational stage is:
Birth to 18 months
12 years onwards.
2-6 years.
7-12 years.
According to Piaget Cognitive theory, the sequence of development has been categorized into 4 major stages:

Sensorimotor stage (0 to 2 yrs)
Pre-operational stage (2 to 6 yrs)
Concrete operation stage (7 to 12 yrs)
Formal operation stage (11 to 15 yrs)