At what age is bronchiolitis (inflammation of the small airways) most likely to occur?

Age Distribution of Bronchiolitis

Bronchiolitis most commonly occurs in infants under 1 year of age, with the highest risk and severity in those younger than 12 weeks (3 months). 1

Primary Age Range

• The American Academy of Pediatrics defines the target population for bronchiolitis as children from 1 month through 23 months of age. 1
• Bronchiolitis is the most common lower respiratory infection in young children and the leading cause of hospitalization in those aged less than 1 year. 1
• Approximately 90% of children are infected with RSV (the most common cause) within the first 2 years of life, with up to 40% developing lower respiratory tract infection during their initial infection. 2, 1

Peak Incidence and Severity by Age

Infants under 3 months represent the highest-risk group:

• Most hospitalizations for bronchiolitis occur in infants under 3 months of age. 3
• More than 50% of deaths from bronchiolitis occur in patients younger than 3 months. 3
• Infants younger than 12 weeks are at significantly higher risk for severe disease requiring hospitalization. 1

Age-specific clinical patterns:

• Infants 1-12 months present with more pronounced increased work of breathing, including tachypnea, nasal flaring, and intercostal retractions, and have higher risk of apnea (particularly those under 1 month and premature infants). 1
• Children 12-24 months typically present with rhinitis and cough progressing to wheezing and rales, but generally compensate better for increased respiratory effort than younger infants. 1
• The average age in clinical studies is approximately 10 months, with severity decreasing as age increases. 4

Epidemiologic Context

• RSV bronchiolitis leads to more than 90,000 hospitalizations annually in the United States, with the vast majority in infants less than 1 year old. 1
• The hospitalization rate for children under 1 year is 3,838 per 100,000 healthy children. 3
• The cost of hospitalization for bronchiolitis in children less than 1 year old exceeds $700 million annually in the United States. 2

Critical Clinical Caveats

Neonates (under 1 month) require special consideration:

• These infants fall outside the standard AAP guideline age range but can develop bronchiolitis. 1
• They have substantially higher risk of severe disease, apnea, and serious bacterial infections (approximately 10% risk of SBI). 1, 5
• Febrile neonates with bronchiolitis symptoms warrant more aggressive evaluation for concurrent bacterial infections. 5

Children outside the typical age range:

• While bronchiolitis can occur beyond 24 months, these children may have different clinical presentations, management needs, and outcomes. 1
• Children with persistent symptoms beyond 4 weeks may represent "post-bronchiolitis syndrome" rather than acute bronchiolitis. 1