I looked. I knew it was the aspen or the fungi in Michigan.
Ok, it may not be the oldest, but it may be close.
This is not what I was thinking about.
hXXps://www.scientificamerican.com/article/strange-but-true-largest-organism-is-fungus/
This:
https://www.smithsonianmag.com/smart-news/mushroom-massive-three-blue-whales-180970549/