To the first two statements: Well, is he right or wrong? The first statement can be checked by looking at the literature. For the second statement, Stallman himself is in the best position to judge that. The third statement is certainly rude, insulting, and uncharitable (it's not true-false so can't be judged that way). The fourth statement is, technically, an argument about a definition, though it suggests a view I shall address below.

The problem about child sex and the age of consent is this: Some people are "ready" for sex (in the sense that it won't be traumatic—or is unlikely to be traumatic, anyway; I suspect it's a matter of probability, never of complete certainty) at a certain age, while others are not ready even if they think they are. I'm sure there are 22-year-olds who aren't ready for sex, and 12-year-olds who are. Unfortunately, I haven't heard of any psychological test to determine whether someone is ready.

(Which isn't to say that such a test is impossible. In fact, if there were something analogous to "driving school" that taught people the basics of sex and gave them some practice—run, of course, by consummate professionals—I suspect that would be a much better way of helping people mature sexually while minimizing the risk of trauma. But that seems unlikely to be politically feasible.)

So instead there's this "age of consent" thing, which just sets an arbitrary age limit as an approximation—obviously arbitrary in that it varies from place to place. This fails to protect those who are immature for their age, and it is a violation of the autonomy of those who mature earlier. (For illustration, imagine if my middle school science teacher had her way—"No sex 'til you're 25", because that's when the brain's frontal lobe finishes developing.)

Some people think this is an acceptable tradeoff. Others reject it, perhaps considering it unprincipled and unjust. "Just because some people in a group can't handle it, that doesn't justify banning it for the entire group." (For example, some adults over 21 clearly can't handle alcohol—get addicted, become violent drunks, and end up ruining their own and others' lives—yet I don't hear people advocating the return of Prohibition.)

Characteristics that would make someone likely to object to age-of-consent laws: (a) maturing early, putting them on the "it violates my autonomy" side; (b) caring strongly about people's autonomy in general; (c) caring strongly about principles; (d) subscribing to a modern, left-associated "sex really isn't that special; norms against it probably come from religious prudes from past generations" view; (e) not being shy about holding and expressing controversial opinions. I think the above explains how Stallman came to his views (as well as some others I won't name).

As for how someone might reject those views—well, Stallman himself said: "Many years ago I posted that I could not see anything wrong about sex between an adult and a child, if the child accepted it. Through personal conversations in recent years, I've learned to understand how sex with a child can harm per psychologically. This changed my mind about the matter: I think adults should not do that." I might add another factor: some people, especially children, are very passive and vulnerable to social pressure, especially from adults, and from those people a "yes" may not mean very much, nor can they be trusted to say "stop, I don't like it anymore"; obviously Stallman is extremely resistant to social pressure, and I suspect he found it hard to take that scenario seriously (if it even occurred to him) until someone gave him real examples of it.