How far back does this data go?

[papermache]

What would be the oldest 990 data I can grab?

[natmey]

There are lots of sources. ProPublica's Nonprofit Explorer has indexed them going back to 2011, but has PDFs available all the way back to 2001. They also have an API.

Whether or not the PDFs are helpful probably depends on whether you're going deep on a couple of organizations, or going wide across a subset.