Why Is Adult-Onset Asthma More Severe Than Childhood-Onset Asthma

Your question: Is it known why late-onset/adult-onset asthma harder to treat and control?

My humble answer:  I have done lots of research into differences between childhood and adult-onset asthma. While there may be a variety of theories (guesses) explaining this, I think there is one that sums it up best. One theory says that the differences in childhood and adult onset asthma is the type of airway inflammation that is present. A few years ago, one allergist told me that nearly 100% of childhood-onset asthma is allergic, and so they tend to have Th2 inflammation (https://asthma.net/living/th2-dominant-asthma). And this type of inflammation responds very well to traditional asthma medicines (bronchodilators, corticosteroids, etc.). Those with adult-onset asthma tend to have different types of airway inflammation that tends to be stubborn and more resistant to traditional asthma medicines -- making them more difficult to control. Good examples here are eosinophilic asthma (https://asthma.net/living/persistent-eosinophilic) and neutrophilic asthma (https://asthma.net/living/subgroups-the-basics-of-neutrophilic) -- both of which are more likely to occur in adults than children. Actually, another article you may find helpful is this one (https://asthma.net/living/subgroups-the-basics-of-neutrophilic).