@ Gerard
A very good question -- but a huge can of worms !
I've tried get my hands on as much source material as I can as my profession is dealing with statistics. From what I've read, the data is all over the place for Keratoconus. If you're interested in the subject, The Review of Ophthalmology
http://www.revophth.com/ is an excellent source of published research info. What seems to occur time and time again is that, no matter what you're talking about (be it crosslinking, intacs, grafts of whatever flavour, age of onset etc. etc. etc.) there's simply no such thing as typical Keratoconus. You can make some sweeping generalisations, like it is usually clinically apparent first in your teenage years or that the thinner your cornea the more difficult it is to get a consistent refraction and the worse your vision. But when you get down to the individual level, the generic "rules" break down too often for them to be a reliable universal read-across.
For instance, you can get a not-too-bad corneal thickness of, say, 500 microns in a Keratoconic cornea but really lousy vision. Or you can have a 400 micron cornea i.e. pretty thin and the person hasn't noticed anything wrong (nor has their optometrist because they usually don't perform that test) until they consider LASIK or similar and it's picked up then.
It might also be the case that even if some of the working assumptions are more robust than others, because of the rarity of the condition, there's too much noise in the data due to the necessarily small sample sizes.
If any of our optometrist friends has any never-fails (or, maybe, usually-never-fails) observations, things that are always true for Keratoconus, would be fascinated to hear them. Or, indeed, on the original question about continued progression at 40+ or late onset Keratoconus.
Cheers
Chris