No, you have to be older than 21 years to write the adult matric exam (the amended senior certificate). There’s nothing stopping you preparing for the matric exam by studying. Remember that the matric exam takes into account years 10 to 12 of studying, so you may have quite a bit of catching up to do. If you are wanting to write the adult matric exam, you need to make sure you have at least grade 9 and are older than 21 years when you write the actual exam.