Dear Student,

It is very okay to join a UG course at the age of 21.

In India, there is no age limit for joining a Under Graduate course and also for completion of it. You can do it if you are less than 21 years old too or even more. Though in some special UG courses like MBBS, a person is not allowed to pursue it after 30 years of age.

Exams like JEE MAIN and Advanced also do not not have any age limits but after completing your 12th, you get only 3 consecutive years to sit for JEE MAIN and 2 consecutive years for JEE Advanced. But then again, you can sit for other engineering exams and take admission in privates colleges at any age.

I hope this answer clears your doubt.

All the very best!