Hello aspirant, according to eligibility criteria for BISTAT 2021 since you have qualified class 12th in 2020 you are eligible for BITSAT 2021 exam if you have minimum 75% in aggregate in Physics, Chemistry and Mathematics . Since you failed in 2018 class 12 exam you are not required to fill details of 2018 but you must enter latest class 12 passing details while filling the application form. The last date to fill the application form is June 30. There is no age limit provided under BITSAT 2021 eligibility criteria.

For further details on eligibility of BITSAT exam visit -

BITSAT 2021 Exam - Notification (OUT), Registration (Started), Pattern, Syllabus, Admit Card (careers360.com)

To predict which branch and college you can get according to your marks in BITSAT exam visit - BITSAT College Predictor 2021 - Predict Colleges based on your Rank (careers360.com)

BEST OF LUCK!!