Step 1: Understanding the Concept:
IMNCI uses simple clinical signs to categorize the severity of illness in children.
Fast breathing is a key indicator of pneumonia.
Step 2: Detailed Explanation:
The threshold for "fast breathing" depends on the child's age because normal resting respiratory rates decrease as a child grows:
1. Age < 2 months: $\geq$ 60 breaths/min.
2. Age 2 months to 12 months: $\geq$ 50 breaths/min.
3. Age 12 months to 5 years: $\geq$ 40 breaths/min.
Since the question asks for a child aged 12 months to 5 years, the threshold is 40.
Step 3: Final Answer:
The threshold for fast breathing in a child aged 1 to 5 years is $\geq$ 40 breaths per minute.