I cultured PA around 6 years old and wasn't hospitalized for it until I was around 9 years old. I think it mostly has to do with how you're feeling. The fact is, PA probably won't ever "go away" (sorry, i know that's upsetting to hear) forever, but in the clinics I've been to, they've only treated it if its causing problems (which it sounds like Ashton may be having with his cold). So, basically, what I've been told is they will treat PA if it is causing an exacerbation, but may or may not actually "get rid of it" for awhile. If he's just culturing PA, but is having no problems (can still gain weight, exercise, sleep well, feel well, etc.), they are likely to leave it alone. That is what my docs have always done.
Please know I'm thinking of Ashton and you. I'm sure this is very tough to deal with right now, but know its not anything out of the ordinary, and the fact they caught it early on and are starting some proactive measures such as TOBI is great. Everything, when caught early, is better able to be treated by whatever method. Please keep us posted.