4. After what age is it extremely uncommon for an infant to develop congenital hypertrophic pyloric stenosis? Posted on April 19, 2023 Correct answer: After 12 weeks of age it is extremely uncommon. Post navigation Previous Post3. What percentage of cases of congenital hypertrophic pyloric stenosis occur in the first born? Next Post5. How does an infant with congenital hypertrophic pyloric stenosis present? What are the clinical features of this condition?