Pure speculation/my own head-cannon - especially since I believe the supposed "evidence" on Mars was ultimately proven to be fake in game:
Dragons, while magical in nature, actually evolved on Mars with its much lower gravity enabling larger flying beasts. When they became sentient they eventually gained full control over their magical abilities and, being magical super-predators, dominated the planet and eventually worked out a civilization not too dissimilar to what is known of them today.
Being the dominant species with huge magical potential and an equivalent ego, of course they would eventually poke behind one curtain too far... While this did lead them to discover the meta-planes, it also ended up drawing the attention of the Horrors. Powerful as they are individually, they have always been few in numbers and even if it took most of the mana cycle the Dragons were eventually almost completely overwhelmed.
In the end, a few managed to open a pair of dimensional gates that allowed them to physically travel to their sister planet (Earth). The creation of this bridge, or perhaps as part of a dooms-day type operation hoping to sever the magical link the Horrors had to our solar system (or both), caused the complete destruction and eradication of the Martian bio-sphere thus rendering it a magically inert rock.
Thus, the 1st Age of magical ebb is actually the 1st Great Sleep of the Dragons after fleeing their homeworld. Before that was only the 'Golden Age' of life on Mars.
The 2nd Age would be the Horrors attempting to reconnect to the Dragons and the new world - perhaps they managed a scouting party but were largely driven off because they could not find the Dragons before the mana began to ebb again. This is why the Dragons of the 4th Age encouraged/aided metahumanity to build the ... forget the name for them, the arks where they hid until the horrors went away... Cairns? They also (from what little I know of ED) helped to raise and rally armies of mages and adepts in order to fight the Horrors, again with little to no success, because when it comes to Magic the Horrors are lit AF.
Finally, in our current 6th Age - in the absence of magic metahumanity has embraced technology, which many Dragons seem to view with great interest. The Big D (and possibly others?) encouraged the development of techno-magical devices, possibly with the hope that a blending of magic and machine might be enough to destroy the Horror's once and for all.
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Again, all just stuff from my head, but hope it helps your own thoughts