In JEE MAINS there is an age limit to apply for Btech course. 

Students who are born on or after October 1, 1993 are only eligible to apply for JEE Main.

But to apply for btech, you should have PCM/B in your 12th. And if you don't have these courses then you are not eligible for btech or btech related courses like biotechnology. Some engineering exams do not have any age limit mentioned. But first you can clarify your 12th subjects and stream and accordingly I will guide you.