Yes you can study btech at IIT at the age of 21.

In order to be eligible for the JEE Advanced attempt limit, candidates must have been born on or after October 1, 1999.
For candidates who are SC, ST, or PwD (born on or after October 1, 1994), there is a five-year relaxation.

For more information, please visit the following link:

https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria

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Yes, you can study at IIT for BTech at the age of 21. There is no upper age limit for admission to IITs. However, you need to meet the eligibility criteria and qualify for the Joint Entrance Examination (JEE) Advanced, which is the entrance exam for admission to IITs.

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