Is there any age limit for the ICRB exam . l came to know that the isro takes scientists directly from IIST . if I am studied at any other college will they give me importance?I am so confused

The best option for you is to join Indian institute of space and science technology. If you have score of cgpa 7.5
You have a very good chance for absorption into ISRO directly from IIST rather sitting ISRO usual intake procedure.
The best thing about IIST in this aspect is that the courses(Aerospace/ Avionics/ Engineering Physics)are improvised to meet the requirements of ISRO, so you will need less time to sink into the Organisation.

It helps you in experience in all the aspects of  take iist and to join in ISRO the possibility is high it helps at btech level you get a chance to do the internships and some other projects in  various ISRO centers which help you to become a ISRO aspirant.

You can also come through IIT and nits for this and later on sit in ISRO intake procedure i think you have got the difference.
Best colleges are:

Indian institute of space and science technology
Indian institute of remote sensing
Physical research laboratory etc

So don't be confused read above matter carefully think we'll about it go further.

Age limit is you should be below 35 years . Here you should be below 15 January 2019 up to this date then you are eligible for the exam.