Four months before your 65th birthday. Go to the Social Security site and look under FAQs in the top right corner. Taken for the Social Security site: How soon can I apply for retirement benefits Updated 05/06/2011 10:15 AM | ID# 1845 How far in advance can I apply for my Social Security retirement benefits? To apply for Social Security retirement benefits, you must be at least 61 years and 9 months of age and want your benefits to start in the next three months. Even if you are not ready to retire, you should still sign up for Medicare four months before your 65th birthday.