I have to pretty much ditto a lot of what Melissa said -- it's so hard to give you a clear cut answer. We have all asked ourselves and our doctors that same question and nobody can tell you for sure. It is very much a "by the day, month, or year" at a time kind of thing.
But having said all of that, it always "seems" to me that the longer you go with the least amount of symptoms is just better all the way around.
You can check out my blog page for the full story and a recent update, but the short version goes like this:
My kids were late diagnosis too. My daughter at age 14. My son at age 18.
They are both pancreatic sufficient.
My son is virtually symptom free still at age 20 (although tests do show some lung issues, they aren't anything to get too excited about and he does not do any treatments)
My daughter has the best lung function at age 18 than she has ever had (almost all her numbers are above normal) thanks to Pulmozyme, Albuteral, 7% Saline & The Vest. Along with a lot of hard work on her part to keep herself healthy (learning not to over do things, getting plenty of rest, using hand sanitizer so she doesn't pick up a cold virus from others etc.)
While this may not be accurate for others (and may not even come true for us) our doctor did make the statement (and yes he gave us the disclaimer first) but he said when she was 14 years old, we might be measuring the progression of her disease in "decades" rather than "years" like they sometimes see in the more "classic CF" cases.
Since her diagnosis 4-5 years ago, I'd have to say he has been correct so far. Her lung function has increased, not decreased and we have not had a "change of status" yet.
From what I read about many people on here and on another website, it seems that many people say they were in really good shape most of their childhood and early adult life - it was when they got to their 20's or 30's when they started to see a larger change - some people say they feel it happened when a combination of things changed in their lives, like going to college, burning the candle at both ends, quit exercising, slacked off on meds etc. Other's say nothing changed, they just got slammed with problems even though they were doing the best they could.