Feline asthma is relatively uncommon in younger cats, and that's part of the reason it took so long to diagnose, it was far down on the likely list of issues. This little guy had a very hard start on life, found as a kitten lying in a ditch, too young to be away from Mama, with fang punctures on each side of his neck. He was diagnosed with asthma at about 3 years old, along with yeasts, molds and a couple of mycoplasmas in his lungs that also had to be evicted.
The old boy (15 year old coal black DSH) either has chronic inflammatory bowel disease, or small cell lymphoma in his GI. I have an entire page hanging on the fridge with his meds schedule. Treatment for both is more or less the same, but the steroid (that he's no longer on,) caused some serious thoracic and heart issues. We don't expect him to be with us as long as we want, we'll take every day we get.