How old were you "SUPPOSED
Most doctors don't share this kind of information to "play god" or tell you when you're going to croak. Not everyone is going to think to ask "Excuse me, will I die young?" but you can bet that the majority of people will want to know (majority wanting to know mean doctors learn to spread that information to anyone new they come across) if they can expect a shorter lifespan. When people get sick with something, or first hear a diagnosis they don't understand, that's usually one of the first questions. "Am I going to die?" "How long have I got?" etc etc. And doctors are going to spread the information at their disposal (which will be the averages at the time). Plus, quite frankly, it's their job and responsibility to spread any information they have at their disposal about your condition. An average life span is just that -- an average. But it's fairly important information that they have that they should be passing on. If my doctor had kept information from me because it was an "average" and "not set in stone" I'd be pretty pissed, because they would've left out a whole load of CF information that's valuable. A whole lot of medical information in general is based on averages.
Allie's Ry wasn't originally planning in going to college for that same reason. His parents told him this was not acceptable, and that if he wasn't going, he was going to move out. He kept putting off applying. He came home one night in the middle of summer, still having not applied, to find ALL of his crap in boxes on the front lawn. He registered for college classes the next day, because his father wouldn't let him back in the house until he actually had a receipt for it.
If you were choosing to ignore your schoolwork, that's a decision you made. Unless your doctor grabbed your shoulders and said "You will NOT live past 18! Listen to me! Accept it!" then s/he really carries no blame in the situation. It's something you and your parent(s) didn't do properly. Everyone makes mistakes, but it is not the doctor's fault that you didn't take care of your responsibilities. S/he was making good on his title, and sharing the medical information s/he had at hand on CF, which is what s/he should be doing. <i>(This is, of course, assuming s/he didn't do as I said above and try to convince that you WOULD NOT live past 18. If s/he did that, s/he did it incorrectly. Doctors tend to present it much softer than that, though. Like Debbie said, they tend to even be <b>too</b> vague, not wanting to say anything is a fact because science is constantly being updated. So it usually comes out like "Current average is 18." "You don't have the best chances of living past 18." etc and that's fine, that's just them spreading the information they have, not telling you when you will or won't die).</i>