oldest found jain idol

The oldest widely accepted Jain sculpture found so far is the Lohanipur torso. It is a polished sandstone statue dated to roughly the 3rd century BCE to the 2nd century CE and is believed by many scholars to represent a Jain Tirthankara. The statue was found in Lohanipur, Patna (modern Bihar). This makes it among the earliest known Jain images discovered in India. There are other ancient Jain legends about very old idols, but the Lohanipur torso is the earliest concrete archaeological find commonly cited by Jain and archaeology scholars. Different Jain traditions may discuss earlier legends, but the Lohanipur find is the oldest material artifact with scholarly dating accepted in mainstream research.

Note: Some stories mention very ancient idols (e.g., Antarikṣa Parśvanātha) in devotional literature, but these are not supported by the same kind of archaeological dating as the Lohanipur statue.

You can read more about early Jain sculpture and the Lohanipur torso in standard Jain and archaeology sources.