> I'm finding it surprisingly hard to find sources for known age of species - is that information collected somewhere? Or is it something we often just don't know because of how sparse the fossil record is?

It depends on what you mean by the age of the species. You can find the oldest known fossil occurrence at the Paleobiology Database [1] and the divergence time from molecular phylogenies via TimeTree [2].

[1] https://paleobiodb.org/

[2] https://timetree.org/