NDA exam is conducted for the admission of the candidates in National Defence Academy and Naval Academy. So in order to appear in this exam you need to fullfill one of the mandatory criteria i.e your age limit should be in between 16.5 to 19.5 years and must have passed 12th. So your age in 2021 will going to be 19 years in month of May .Therefore , you can take nda exam in 2021 as it is mostly conducted in April and you will satisfy the eligibility.
Moreover you can check more by below link.
To be eligible for NDA-1 2020 a candidate must have be born in between July 2, 2002, and July 1, 2005 and the age must be in between 16.5 years to 19.5 years. According to the eligibility criteria you are not eligible for NDA 1 2020 as you are born before July 2, 2002. Please click the link below to check the eligibility criteria
According to the eligibility criteria of NDA, only students having tha age between 16.5 years to 19.5 years can give NDA paper. Now your current age is 18 years and 4 months which lies between 16.5 to 19.5 . So you can surely give NDA exam.
Also there are some other eligibiltiy criteria for NDA paper and other necessary things that you must know. You can check all these things by clicking the link given below.
Hope it helps.