The oldest dynasty in CK2 are the Cholas in South India. They are first mentioned in the inscriptions of Ashoka in the 3rd century BCE, and are probably even older than that. Unfortunately, we do not know exactly how old, because the oldest (concretely dated) written inscriptions from India are in fact those same Ashokan inscriptions.
Pandya Dynasty is even older. They are neighbours and historical rivals (and allies) of Cholas, and they are known and written about in literature as early as around 600 BC. And they remained more powerful than both Cholas and Cheras for almost a millenia.
In general, Pandyas, Cholas and Cheras all have their dynasties going well back into the BCs.