You can actually develop asthma at any age. Sometimes it might be just that a person has had mild asthma their whole lives yet haven't had symptoms of it.  And perhaps the person is exposed to an allergen and developed a sensitivity to it over time, and thus asthma shows up. 

So, yeah, it's kind of a strange disease this way.  Ot's also common for asthma to show its ugly head and then simply disappear.  

If asthma occurs at this age I'd wonder if the person is exposed to something she wasn't exposed to on a regular basis before, such as dust or cockroaches or a new type of tree.  The asthma might resolve itself once the irritant is removed.  Or it might simply be bum luck.

Regardless, asthma can be controlled and the asthmatic can live a normal, active life with the disease.  It's simply a matter of being asthma smart and working with your doctor.