There are two different ways to look at this question:

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1. According to Jain traditional belief (śāstriya dṛṣṭi)

From the point of view of Jain granth and tirth-māhātmya, many idols of Bhagwan Pārśvanāth are described as anādi / very-very ancient, not just “a few thousand years”.

Some famous examples:

• Śaṅkheśvar Pārśvanāth (Gujarat) – Very highly praised in Jain tradition; many texts and elders describe this idol as extremely ancient, existing from very early times of this avasarpiṇī. ( en.wikipedia.org)
• Antarikṣa Pārśvanāth (Shirpur, Gujarat) – In Vividha Tīrtha Kalpa it is said that this idol is the third-oldest idol in the universe, after:

1. Bhagwan Nemināth’s idol at Girnar, and 2. Bhagwan Pārśvanāth’s idol at Śaṅkheśvar. ( en.wikipedia.org)

So, as per Jain faith and tradition, the pramukh “oldest” Pārśvanāth idol is generally taken to be Śaṅkheśvar Pārśvanāth, with Antarikṣa Pārśvanāth also described as extremely ancient.

(These are matters of āstikya–śraddhā based on Jain texts, not on modern scientific dating.)

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2. According to modern archaeology / history

From the side of historians and archaeologists:

• The earliest securely dated images of Pārśvanāth are from around 2nd century BCE to 1st century CE.
• Important early examples include:

- A stone āyāgapaṭa with the image of Pārśvanāth from Kankali Ṭilā (Mathura), now in the State Museum, Lucknow, dated roughly to the 1st century BCE–1st century CE. ( en.wikipedia.org) - Early bronze images in museums such as CSMVS (Mumbai) and Patna Museum, dated around 2nd century BCE–1st century CE. ( en.wikipedia.org)

Archaeology keeps changing as new excavations happen, so no one can say with 100% certainty which single idol is the absolutely oldest in the whole world.

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Simple summary

• In Jain śraddhā and granth-paramparā:

The “oldest” Pārśvanāth idol is accepted as Śaṅkheśvar Pārśvanāth, with Antarikṣa Pārśvanāth also described as one of the most ancient idols in the universe.

• In modern historical / archaeological terms:

The oldest known images of Pārśvanāth are from ancient Mathura and similar sites, roughly 2nd century BCE–1st century CE, but scholars cannot point to one single idol and say “this is definitely the oldest in the world”.

So, for a Jain believer, the answer usually given is Śaṅkheśvar Pārśvanāth Bhagwan (and Antarikṣa Pārśvanāth as one of the oldest), while historians speak more carefully in terms of the earliest known images, not a single oldest idol.