My suspicion is that you may be holding "informed decision" to an unfairly high standard. Granted, we often do not make "informed, prudent decisions." We human beings are certainly not omniscient, and we sometimes reason badly. But plenty of decisions we make can be made rationally without being omniscient: I don't have to know all that much to (say) choose between a salad and an omelette for lunch, or to decide whether to have a summer or a winter vacation this year. Of course, some decisions are more complicated. But I'm happy to say that we have enough knowledge in many cases to make decisions that meet minimal standards of rationality.
You may also be making the controversial assumption that the only factor relevant to "age of consent" is rationality. Ethically speaking, certain decisions matter a lot to us because they impinge on fundamental concerns. Whom to marry, for example, is not a decision that others should make for us. For one, even if I'm not perfectly rational about that decision, I'm still likely to be better situated than anyone else to make that decision. Moreover, I'm might care that the decision be left up to me, even as I recognize I may make a bad decision. In other words, I want the right to be wrong in certain decisions I make. That idea has been a prominent theme in the work of anti-paternalist philosophers (e.g., J.S. Mill).
So no, I don't think age of consent is "meaningless" even if we acknowledge that human beings often don't make the most informed or prudent decisions on their own behalf.