Final answer:
Blood pressure is usually not assessed in children younger than 3 years due to the ongoing development of their cardiovascular systems.
Step-by-step explanation:
Blood pressure assessments are typically not conducted in children who are very young because their cardiovascular systems are still developing. Routine blood pressure monitoring usually begins after early childhood. Understanding the age at which blood pressure is typically measured can help identify potential issues early on. The information provided suggests that blood pressure tends to remain more or less constant after the age of 20 years in non-modern communities. Additionally, infants may have a lower blood pressure due to various physiological factors including the shorter distance to which blood must be pumped.
Given the context, the correct answer is: 3 years, indicating that blood pressure is usually not assessed in children younger than that.