If You don't satisfy the eligibilty criterion it means You can not appaer for the exams. You should however check the date that on which data they are demanding You to be exact 18 yaers old, It may happen that stil You are not 18 on the day of Exam but You may be eligible for the exam, If they did't mention any such conditions tha=en You should ask them Byu writting a mail to get confirmed and Not to aniticipate anything beforehand, Perhaps If You don't satisfy the eligibility critertion No one will allow You, But You should be very accurate about the eligibilty criterion.