There is not a default timeline. This isn't an accident. 13th Age sets itself up to only give half the story. The reason for this is because the players are encouraged to affect the world via their One Unique Thing, and just general backstory. For instance, one player in a game had, "I look just like the Emperor's only son." Obviously there's a reason for this, so now either the Emperor has an illegitimate son, or maybe there were twins that got separated at birth. You'll find out later.
There are a handful of specific events that we do know though, But we don't know which age they happened in. They are, in their entirety (so far as I know):
If these happened in the 12th Age:
- The Lich King was not yet undead, and was the Wizard King. As the Wizard King, he killed one of the other Icons of that age, The White (dragon).
- Ultimately, the previous Orc Lord killed the Wizard King, which was the ending of that age. Since there's a new Orc Lord in this age, both of them were probably killed together.
If they happened in an earlier Age:
- The Lich King was still not yet undead, and was still the Wizard King. He still killed one of the other Icons of that age, The White.
- Ultimately, the previous Orc Lord killed the Wizard King, but most likely didn't die. We don't know how long each age was, and how long orcs live, but since they generally die in battle, and this orc is the Orc Lord, he could have lived a long time indeed. That doesn't make this age being the first or second age very probably still, but possible. More likely it was the 8th or forward, if each age is a good fifty years or so. The 13th Age has lasted at least that long.
p.s. The Leviathan is mentioned only once, as a possibility, and the Green is held by the Priestess, but for an inordinate amount of time. There's no way to date those at all, even suppositionally.