Hello vanita!!Till last year, a candidate had 3 attempts to appear for exam.But,as per the latest updates there is no limit on the number of attempts  candidate can take up.A candidate must have 17 years of age on and upper age limit for general category is 25 years and 30 years for candidates from OBC/SC/CT categories. just go through the below link to have a clear view:

https://medicine.careers360.com/exams/aiims-nursing

hope this would helps you.Good luck!!