From the Jain point of view:

• Jain Dharma is anādi–anant – beginningless and endless.

It is not “started” by any founder, so in Jain belief it is eternal, beyond comparison like “older” or “younger”.

• In every time-cycle, Tirthankars (like Shri Rishabhdev/Adinath, the first Tirthankar, and Shri Mahavir Swami, the 24th) re-discover and preach this eternal path. So Jainism itself is not considered new or recent at all.

From the historical / academic view:

• Historians usually talk about “oldest religion” using written records and archaeology.
• There are several very ancient traditions (Vedic/Hindu, Jain, Buddhist, Zoroastrian, Jewish, etc.).
• Because of this, scholars generally do not give one single religion the clear title “oldest”, and different criteria give different answers.

So, you can say safely:

• According to Jain Dharma: Jainism is an eternal, timeless dharma.
• According to historians: Jainism is one of the world’s oldest living religions, but not uniquely labelled “the oldest” by everyone.

You can read a simple explanation here