From a Jain perspective:

• Dharma is beginningless (अनादि). Jain scriptures say that true religion – the path of right faith, right knowledge and right conduct – has no starting point. It always exists in the universe.
• Jainism is called an eternal tradition (शाश्वत धर्म). The 24 Tirthankars of each time‑cycle do not “found” a new religion; they re‑discover and re‑preach this eternal Dharma. So in Jain view, Jain Dharma is not “started” by any human at a particular date.

From a historical / academic point of view:

• Scholars usually say: we cannot prove which single religion is the “oldest,” because written records are limited and many traditions are very ancient.
• They generally agree that Jainism is one of the oldest living religions of India, with roots going back at least to the time of the early Vedic period and possibly earlier, but they avoid calling any one religion the oldest with full certainty.

So:

• Jain view: Dharma (and thus Jain Dharma) is eternal and beginningless.
• Modern history view: Jainism is among the world’s oldest surviving religions, but historians do not give a definite “number 1 oldest” label to any one faith.

You can read more about Jain Dharma here: