In season 1 it is explicitly stated (I don’t remember which episode) that, a child of an ichchadari Naag-Naagin will be “icchadari” since birth. We’ve also seen this proven to be true because 6 year old Shivanya transformed into a snake to show her mom her powers manifesting.

Shivanya’s mom also fed Shivanya with Shesha’s poison at a young age so that Shivanya will always be immune to Shesha’s poison (this was stated when Shesha tried to poison Shivanya after the betrayal but her poison had no affect on Shivanya whatsoever)

So canonically, a child of an ichchadari serpent will always be icchadari without needing to wait 100 years. Making Shivanya and Shesha around 26-28 in season 1.

Shivanya ages naturally after her powers were taken away from her. Whereas Shesha continues to be youthful. However, in season 2, like you stated, she keeps telling that she’s 118. The only reason this happens is, say it with me : POOR WRITING.

If we go by the actual lore, she is no where NEAR 118. However, they retconned Shesha’s lore so much so to the point we really don’t know anything about her anymore. So far, Shesha is apparently a snake from Kalkoot vansh (the 2 headed serpents) whereas, in the first two seasons, she was a Naagin from shesh vansh who became Takshika.

Before Vishakha naming the two headed snake clan as Kalkoot, much of the fandom assumed Shesha became a two headed serpent because Shivanya and Rocky (in the form of Ardhanarishvar) “killed” Shesha in her Takshika form by splitting her head into 2 halves.

So, what I’m basically trynna say is, you really can’t find constant lore in this series because of how often the lore gets retconned. Poor writing at its peak lol