Where does it say they have to be old, and how do you know it isn't a lump sum? Sure sounds like it is to me from the article.
Because you have to be old to get Social Security retirement funds, and both Mexico and the U.S. pay out monthly, not in lottery lump sums (with some exceptions for mental retardation, etc.). Also, note that the article says that they have to have first paid in to Social Security. Most illegals are paid in cash, and make no such payments to our Social Security Administration.