According to a doctor I spoke with recently 80% of people with CF are diagnosed by age 3. Among that group, about 50% will live to be over 25. (Keep in mind that life expectancy is a moving target -- it's constantly improving due to to new therapies, etc. Kids born with CF today are different than a child born and diagnosed early in the 80s. Also, more children are being diagnosed early now, regardless of disease severity, due to things like newborn screening, so this "25" number is going to change, and doesn't mean much anyway.)
An additional 10% of people with CF will be diagnosed by the time they hit 18. And then it's something like 5% of cases that are diagnosed after that as an adult. According to the doctor I was talking to, the left over percentage is people who are never diagnosed because they either have a benign mutation and never show ANY symptoms or the diagnosis is simply missed.
The doctor I was talking to was in fact a CF doctor (adult), but this is the first time I had heard those stats. After talking to him I was on my tx center's webpage and noticed they also use the "half of people with CF diagnosed in infancy or young childhood will survive past their late 20s" statistic, although they also say that number is increasing. Crazy.
Anyway, point is that even though only a small portion of cystics are diagnosed as adults, it definitely happens. There are plenty of people on this board who were diagnosed later. My guess is that people who receive a later in life diagnosis are more likely to seek support to get them through it, though, so the percentage on this board is probably a little skewed. That's totally just a guess on my part.
Good luck and hope you don't have CF. Regardless, welcome!