A comment on “the problem of Susan” and more particularly this post by Adam Roberts:

My thesis: None of the Pevensies goes through puberty in Narnia. Remember, while we are told that they “grow and change,” we don’t learn the specifics of that change — or, more tellingly, the ways in which they don’t change. It is perhaps the environment of a planet alien to the one on which they were born that afflicts them all with a rare acquired variant of Kallmann Syndrome. This is why none of them ever marries: though “all princes in those parts desire [Lucy] to be their Queen,” she refuses them because she knows that her condition makes her infertile, and knows that it is best for each of those princes to marry a woman who can bear him heirs. My thesis also explains why the Pevensies so unhesitatingly leave Narnia rulerless: they know that some plan for the succession has to be devised, but also that no Narnian (unacquainted as that land is with modern Terran medicine) would understand their condition or accept that it cannot be cured or remediated. In these circumstances, the responsibility for self-rule is the best gift they could give to their people. Otherwise their abandonment of Narnia would be inexcusable. It is only when they return to our world that they enter puberty for the first time — and puberty is something, I think you’ll agree, that no number of years ruling a country or fighting frost giants could possibly prepare one for.