A slippery slope argument in ethics typically has the following form: If we were to deviate from the status quo in which X is disallowed and instead allow for X, allowing for X (which need not itself be morally objectionable or worrisome) will lead to Y, which is morally objectionable or worrisome. Therefore, we should not deviate from the status quo and allow X. You ask whether the proponent of a slippery slope argument must hold that the first instance is "without much consequence" or if they can instead see it as a "mistake". As the above form indicates, slippery slope arguments generally assume that the deviation from the status quo that 'sets in motion' the slippery slope is not in itself objectionable or worrisome. Dialectically, the point of slippery slope arguments is to concede to one's opponents that the reform in question is not morally objectionable or worrisome, but argue that we ought not to pursue because doing so will lead us down a slippery slope to an objectionable or worrisome...

Your question seems ambiguous between two interpretations: (1) Is it always better to be able to make more choices? (2) Is it always better to have more options to choose among? Under either interpretation, I believe the answer is 'no' -- though the reason is similar in each case. Start with (1): Compare Ann and Bill. In 2015, Ann was able to make 10,000 choices and Bill was able to make 50,000 choices. Is there reason to think that it's better that Bill be able to make more choices overall than Ann? It's hard to see why. For one, perhaps many of Bill's choices concerned trivial matters -- and it can often be bothersome to have make choices about trivial matters. If (like me) you're sympathetic to Kant's view that autonomy (the ability to govern one's own choices or actions, to rationally determine one's ends and how one pursues those ends) is the defining feature of human beings, it doesn't seem necessarily true that Bill is more autonomous by virtue of making, or being able to make, more choices....

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...