i forget mention my date of birth in my decalaration form.. will my application get rejected
Depending on the necessity of the declaration the rejection will happen. If its mentioned in the notice that you should provide DOB then you have to do it and without that you will have greater possibilities of getting your form rejected.
I would recommend you to get it discussed with the concerned authority and they will surely provide you provisions (if any) to correct it.
I wish your problem gets solved. All the very best for your future endeavors!