Avot DeRabbi Natan was written soon after the penning of Pirkei Avot (according to this, it was written from 160 CE to as late as six or seventh centuries).
As far as I'm aware, no manuscript of Avot DeRabbi Natan exists any earlier than the 9th century. What are the earliest/oldest manuscripts for Avot DeRabbi Natan?
If what you say is correct, that there are no manuscripts older than the 9th century, then perhaps this one of version A, originally from the National Library of Russia, thought to be dated to the 9th century. It's just a few fragments, though.
There's also this one of version B, thought to be from the 10th century. Likewise, just some fragments.
I saw on the website of the National Library of Israel that there are multiple manuscripts out there that appear to be undated, so there may be older ones out there.
