Hey Anon,
I think the most important reasons are that it teaches kids how to reason and how to effectively navigate the world using science/logic. Although people “make do” without science education, that doesn’t mean it’s optimal. When you have people denying their children vaccinations or buying ridiculous cure-all scams, these are effects of ignorance which do harm their lives. In addition, it robs their lives of “truth” - that is, what seems to be the reality based on sound arguments and evidence.

Furthermore, the education of the public, aside from being a service to individuals (and it does seem a bit like child abuse to deny children knowledge, but anyways…), it helps the public. The anti-vaccination fanatics are endangering society; climate-change deniers are endangering the planet; when people vote for legislation which ignores scientific facts, they are often hurting others. It’s in the public’s best interest that the public understand science, amongst other things.

As to when to start teaching this? I’d say, the earlier the better, though it of course is dependent on the child’s mental capacity. That said, kids do perform science experiments from the time they’re babies: every time they try something to see what happens, that’s an experiment. Very crude, but science all the same. It seems fairly obvious that the earlier children are exposed to science, the more easily they will be able to understand how it works. (This is aside from the argument that early exposure to science may enthuse kids about science, and the positive effects on individuals and society that that can have.)
A special case could be made for middle- and high-school science education since many young adults don’t attend college, which is where many people “get serious” about science, and is often a prerequisite even for non-science majors. However, many people won’t get that chance (or avail themselves of the opportunity) because they don’t understand what it is that they don’t understand. So early exposure may be the only chance society has to teach these kids. If we don’t, we’re robbing these kids and society at large.

Hope that helps,
-Ari @ RCS