Dear Das, for admissions into any B.Tech course, you need to appear for Entrance Examinations like JEE Mains, JEE Advance,  VITEE, BITSAT, GATE, SRMJEEE, and state based entrance examinations based on the state you are from. For some of these entrance examinations, the minimum age limit is 17 years old. For example, JEE Mains doesnt have any age limit. For JEE Advance  your primary eligibility is that you should have cleared JEE Mains. So depending upon the Entrance examination, the age limit changes. There are few entrance examinations that you can take up which dont have any upper age limit.