MY DOB is 11 August 2008 and I'm currently in class XI can I appear for NDA 1 2027

For NDA, candidates must satisfy the age, educational qualification and marital-status conditions mentioned in the particular UPSC notification.

Since your DOB is 11 August 2008, you should check the exact DOB range given in the NDA 1 2027 notification when it is released. Also, being currently in Class XI does not make you eligible to appear for NDA, because candidates must have passed Class 12 or be appearing in Class 12 for the relevant examination.

Therefore, you cannot apply for NDA 1 2027 while studying in Class XI. You should target the NDA examination for which you will be eligible as a Class XII appearing/pass candidate and whose prescribed DOB range includes you.