sir I have repeted one year so I am back and if i want to give nda exam i should give it before 12 class , after 12 i cant apply for nda because my age limit over. but other children get 1extravyear for appering nda what should i do

As per the eligibility criteria prescribed by the UPSC, final year candidates i.e  the candidates who are going to appear for their 12th board exams are also eligible to apply. So if you think that after completing 12th your age will be over the limited age criteria, then you can appear as a final year candidate in NDA. This is the only way you can appear for NDA. Moreover in NDA there is no age relaxation for reserved category students. So you can't also avail that. You should check the entire eligibility criteria once more before applying http://competition.careers360.com/articles/nda-eligibility-criteria