• Tirthankaras are not born with kevala jnana. They attain kevala jnana (omniscience) after long austerities and spiritual practice, as part of their path to enlightenment. This is the common understanding in Jain tradition for each Tirthankar.

• Some stories mention that at the moment of birth the soul already has highly refined knowledge (in certain narratives or later hagiographies), but the standard, widely-told sequence is: birth → renunciation → long tapasya → kevala jnana → teaching → nirvana. Digambar and Shwetambar texts sometimes differ in small details, but the overall arc remains the same. For Mahavira, for example, kevala jnana is achieved after about 12 years of ascetic practice, before he began teaching. ( jainknowledge.com)

You can read concise, authentic life-sketches of a Tirthankar here:

• Life of the first Tirthankar (Rishabhanatha) and Kevala Jnana path.
• Ajitnath trithankar life (second Tirthankar, Kevala Jnana after austerities).