There is no age to learning, and there are actually many ways by which you can give your class Xth examination. You should enrol yourself in NIOS or the National Institute of Open Schooling. There is no age limit as such and people can attend class Xth or XIIth (or any other class for that matter of fact) at any given age. So, even you are 43 years old, you can easily give class Xth examinations from NIOS and passw with flying colours.

Hope I have answered your question. All the best!