The Last Battle (dismay... I got too old for it, or something. I never liked Aslan though, so I clearly fail as a Narnian.)

No, The Last Battle is just not a very good book. I don't think it's impossible to outgrow because I don't think there's a single age it works for; it's bad sledgehammer theology and the world suffers as a consequence. The actual apocalypse of Narnia is neat. Everything else, DIAF.