An article in this month's Chest (ed Chest? It's an real journal honest) shows that for many patients, asthma never really goes away.
Examining data from a longitudinal study in New Zealand, the study shows that about a 1/3 of patients who were thought to have outgrown their asthma at age 18 had some symptoms by the time they were 26. Fortunately, for most it was mild.
Patients who had asthma throughout childhood which persisted to age 18 were likely to have continued asthma, and it was more serious than in those patients in which it recurred.
Interestingly, about 10% of subjects who had never had asthma, developed it between age 18 and 26.