I do not believe the show ever mentioned Cardassian lifespan, but I've always worked under the assumption that Cardassians are longer-lived than humans--that the lifespan might be below Klingon or Vulcan range, but that they still outlive 24th-century humans quite easily.
OK...lemme dig up the screencaps I was using. I actually spotted this during an avatar contest.
I WILL tell you that Cardassians show age quite subtly until they get older (like, say, about Tekeny Ghemor's age), so it's probably hard to notice until you get still screenshots. Onscreen, during the episode, I can totally understand why you and just about everybody else missed it.
Here's Dukat from the beginning of the episode. This is his standard appearance at "present time" in the series.
http://ds9.trekcore.com/gallery/albums/6x17/deathornight_071.jpg
For a youngish-to-middle-aged Cardassian (and in his late 60s to possibly 70 years old, Dukat would still be youngish by my personal reckoning), you'll notice that the forehead is not perfectly smooth, and that the ridges are starting to become more prominent, but not in the way that you would see with, say, Tekeny Ghemor or Mila.
But here's where I noticed a real change. Here's our young Dukat for comparison:
http://ds9.trekcore.com/gallery/albums/6x17/deathornight_343.jpg
I'm not sure what it is about the Cardassian aging process that does this, but you'll notice that his eye and neck ridges don't seem quite as...prominent. The forehead also seems a bit smoother. There are still a few wrinkles, but I think a little of that is due to the fact that the actor being used was closer to the equivalent of "present-day" Dukat's age and there was only so much they could do to make him look younger. However, there IS a noticeable change in the makeup used for the young Dukat, so I personally contend that he WAS aged backwards, but that Cardassians simply do not show a lot of changes due to age until they reach, perhaps, 80-100 years of age.
It's particularly compelling evidence considering that these screencaps came from the same episode.