Hey buddy, there are three channels via which you can get into IISER and they are-

KVPY: if you have  a valid KVPY fellowship commencing from 2019-20 academic session you are eligible to apply for admissions this year

JEE: if you are in General category securing a rank within 10000 in the Common Rank List  of JEE Advanced 2018 are eligible to apply. if you belong  to reserved category OBCNC, SC, ST, PD then the category rank should be within 10000.(Same Year)

SCB: if you  have passed (10+2) level with science stream in 2017 or 2018 or 2019 with marks equal or above the cut off percentage in their respective boards are eligible to apply.

There is no  age limit, but you can’t take admission if you passed your school three years before the admission date, unless you go through the JEE channel.good luck