CBSE board has not fixed any minimum/maximum age to appear in CBSE Class 10th, 12th examination. However, at the time of admission, the student should satisfy the requirements of age limits (minimum & maximum) as determined by the State/U.T. Government and applicable to the place where the school is located.

Hii!
The minimum age to appear for class 10th exams in cbse is 14years of age as of 31 December of the year of the exam. For class 12th, there is no specifications of age but minimum age is considered to be 17 years of age. Different schools may allow students differently. So, you should be atleast 16- 17 years of age for class 12th exams.
Hope this helps.
Good luck!