I agree with Amanda that it really depends on the person, and it also depends how proactive you are going to be with your care.
I was always pretty healthy, even when I contracted pseudo. at the age of 7.... I had a very normal high school and did very normal high school things <evil laugh>, not paying much attention at all to how my health was or what I was doing. I did however, always do my treatments (cause ma made me!) and take my meds. I rarely missed such things.
Then, in college, I started going downhill fast. I was in a terrible relationship (which I think made everything much much worse health-wise) and also trying to juggle a part time job and full time class schedule. As I got sicker and my FEV1 dropped about 20% and weight dropped about 20 lbs, I cut back on school and focused more on excercise and eating right... and getting rid of the creep I was dating. Without the added stresses of a full school/ work load and a crappy relationship, I just started doing better. Slowly.
Then, after college, I started going to the gym 3-5 times a week and pushing myself on the cardio. I credit this with ALL the difference in my health.... I jumped back up about 10% FEV1 and my weight went up to my high school weight (about 15 lbs heavier!)
So, that being said, I think "turn for the worse" doesn't always mean you can't turn back! I did. I thought for sure my health was going down the toilet. I could see my bones sticking out of my body, could see the bags under my eyes, and could barely walk up a flight of stairs. That's when I knew I had to do something. I would not continue to decline if I had any say in it!
I also think my decision to stop working and stop going to school for a couple years (after I started a masters' program) was the best thing for me. I concentrated soley on my health, and though I didn't want to be limited and unable to finish school cause of this disease, I felt it was more important for me at that time to get to a "healthy place" before I ever go back. I still don't go to school, and am working very part time from home. I feel this has made soooo much difference for me. Perhaps, if I can get my FEV1 way up, and can put on a bunch more weight and muscle, I will go back. But, I personally like feeling awesome, even if it means I have to sacrifice some things I really want. I guess you've got to do what you want to do with what you're given. I turned around and changed my ways in order to live better (and hopefully longer), so 'turn for the worse' is no longer an issue for me.