Follow Your Passion: A Seamless Tumblr Journey
In httyd book 2 we know that Hiccup is 11 because he is no longer 10 but he hasn't turned 12 yet. But in the beginning it says that he's had Toothless for years as in plural which means it's been more than 1 which implies that Hiccup is due to turn 12. Bit since his birthday is in February that means that the events from book 2-5 all happened all within January. So within the span of a month Hiccup almost died to a monstrous strangulator, got kidnapped by Romans, almost died of vorpentitis and surfed down an erupting volcano on a dragon.