I have a good spread across ages. But it does seem like about half of them are below 40.
Viewing post in Cycle of the First Dawn comments
It isn't tied to age. I won't give exact numbers, but the chances deliberately decrease as you find more, so you're not expected to find everything in one stretch. There will be ways to increase the base chances again, and new areas will also give them a bump as they're unlocked. For now there is a practical limit to how much you're likely to find without pushing pretty hard.