Chances are not much high for older IIMS as the weight age is strictly on past academic records as well as other selection rounds like GD/PI/WAT/Extempore,  etc..

So you can even check the complete CAT selection process here.

You will be required to score above 99.6 percentile to get into top IIMs and accordingly will have to focus on your preparation.

You can check the admission process in all IIM's in the link below:

IIM A admission process

IIM B admission process

IIM C admission process

Hope you get the idea of selection criteria in the links attached above.

Good luck.