Then we should assign lower value to people the older they get. Yet it's typically considered worse to murder a very old person than a young adult. Do you disagree?

Personally I don't consider it really worse. In society in general, the murder of an eldery is usually considered worse because the eldery is weaker, but the accidental or "natural" (ie, disease) death of an eldery is considered much less bad than the same death of a young adult.

Everyone is equally unable to defend themselves against a gun, or a Death Eater with a wand. This may be relevant when you're talking about hitting someone, but not for murder.

It is not relevant for the murder itself, but it is relevant overall when considering how society protects people. Large-scale effects are often delt with broad heuristics (like deontology and virtues), and children being defenseless means a deontological injunction "doing harm to children is very very bad" being justified, and that injunction will apply to murder too, even if it's less justified there. Trying to exclude murder from the injunction will weaken it, make it much less of Schelling point, so overall I don't think it's something society should do.