What is the ruling of Islamic law on changing the birthdate in 1987 to evade military service and on the retirement pension that the questioner currently receives as a result?

There is no objection to changing one's age to evade conscription in a war against a Muslim country. It is permissible to receive a retirement pension if its amount is equal to what was deducted from your salary. However, if the pension exceeds what you paid, and the increase comes from an unlawful investment, then you must dispose of it by spending it on the welfare of Muslims. If it is a gift conditioned on a certain age that you have not reached, then it is not permissible to take this increase, and it must be returned. If returning it is not possible, or if there is a fear that it will be spent in a way that is not Islamically permissible, then it should be spent on the welfare of Muslims. If you are poor and in need, you may take what suffices your need and give the rest in charity.