I'm under 18 and do not have any legal ID proof. What ID proof do I present at the counter?

That's a concern, we have that covered. 

Just flash a valid ID proof approved by the Government. 

Your current college/school ID can be your best friend. Just make sure your date of birth and the concerned organization stamp is clear and visible. 

Patrons under the age of 18, who are unable to show the required age proof, can be denied entry by the cinema.