With reference to the two answers provided above, Umesh is correct that you will be appearing for the CBSE Class 10 exam in 2021 and not in 2020.
As far as the specific age is concerned, note that do not go by what is thrown up in a Google search. As rightly stated by Umesh, it varies from State to State. You see, the CBSE Board itself has not fixed any minimum age or a fixed maximum age to appear for the exam of Class 10. The age criteria are determined by (a) the State/U.T. Government and (b) the place where the School is located. [For reference refer to the official CBSE notification: http://www.cbse.nic.in/faq/FAQs_2016.pdf]
So the question is: where exactly are you going to complete your schooling and appear for the Class 10 CBSE exam in 2021? There are also certain criteria for private candidates and attendance and other criteria as well. Hence it would be best if you check the afore-mentioned FAQs as well to see if you fulfill all the required eligibility requirements and not just the one related to age.
Secondly, I am not sure if a medical certificate would be required to prove age eligibility (as mentioned in the first answer). Surely, age eligibility would be proved by documents like a birth certificate.
Your school Principal should know the correct information. If not, I would advise you not to waste time and contact the local CBSE Regional Office where you are going to appear for the Class 10 exam. Please visit the official website http://cbse.nic.in/newsite/contact.html. Scroll down the page to see the list of addresses and contact numbers of CBSE Regional Offices. Check which State is applicable to you as per 'States/UT's/Areas covered'. For example, the Ajmer office is applicable to both the States of Gujarat and Rajasthan. Call the person in charge and make inquiries, politely requesting for quick confirmation as it is urgent. Ask, too, about what kind of supporting documents, if any, are required to prove age eligibility for the exam. Note that you should ask specifically about appearing for the exam in the year you will appear for it and not just ask about going into Class 10.
Please put in a response later and let us know if the issue has been resolved so as to give us also clarity about the age criteria for CBSE Class 10 exam as this page will be helpful in future for students in a similar situation.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell