Ulthuan

I read through the rule book and all of the lore that I could find and this came up.

When did Dragon Princes stop riding dragons and start riding horses?

I believe it was after the sundering but I can't find anything to confirm this.

I believe the Sundering is about accurate.

The Dragon Princes were said to be the greatest warriors in all of Ulthuan who fought Daemons and the forces of the Witch King. Now though Caledor and its Dragons are slowly fading away.

This quote from HammmerWiki does make it seem as though the Witch King was the last big battle that the Princes rode their Dragons to battle.

It would make sense as the vortex drained magic away from the world, magic that sustains the dragons.

Hence dragons went to hibernate, until such a time as magic is abundant in the world again, just before the world is consumed by chaos.

Book 3 of the Sundering goes into it a fair amount. Maedrethnir and the other dragons get a little screen time so to say, with some nice interaction with their riders. Not a bad read over all as are the other two before it.

The answer to your question is in the HE armybook. Check p22, on the reign of Tethlis:

"He sought to end the DE for all time and he might have been successfull had it not been for the decline in power of the dragons. During the latter part of Caradryals reign the dragons had become increasingly rare"

So ballpark figure, -1200 to -1000 on the imperial calendar.

As for the reason, none is given. Most hints point to cooling of volcanoes, disappearance of magic, that sort of thing.

Rod

exactly I dont know when but it is said that in the times of Tethlis, he would have won the war vs druchii if dragons were awake, and in the times of Tethlis they rode dragons, so...I think, maybe after they started to ride elven steeds.

ups, now I see Rod answered you before haha
I don´t remember in which book I read that the heart of Ulthuan was colder and that this was the reason why to see dragons flying in Caledor was really rare...

The High Elves don't know, but have put forward the possibilities listed above.

One of the Dark Elf books claims that the Dark Elves managed to enchant the dragons in order to put them to sleep. Whether this has any truth to it or whether it's just druchii propaganda (we robbed our enemies of their greatest weapon! It wasn't just a coincidence that had nothing to do with us, honessssst!) is anyone's guess.

The simple answer is that they didn't die.

It became increasingly difficult to rouse the dragons from their slumber and the Dragon Princes themselves moved onto elven steeds. I'd agree with rdghuizing's estimation although it's not an on off switch: it was a much more phased thing.

I think the 3rd book ended with the idea that the Dragons were actually wakening up in preparation for the "final battle"