To appear for Joint Entrance Examination, Main, there is no lower age limit as such for the candidates. So, if you have successfully cleared your class XIIth examinations at the age of 15 years, then you can definitely appear for JEE Main and get admission into one of the National Institute of Technologies depending on your performance.

You can also use our college predictor to see your chances in all the colleges:
https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

Hope I have answered your question. All the best!