Final answer:
The predominant developmental characteristic of children at 18 months old is parallel play, where they play alongside others without actively engaging with them.
Step-by-step explanation:
For planning nursing care for an 18-month-old child, it is important to consider the predominant developmental characteristic at this age. The predominant developmental characteristic of children this age is parallel play. This means that children at this age are starting to play alongside others without necessarily engaging with them directly. They may observe others and mimic their actions, but they are not actively engaging in cooperative or interactive play.