How old is Tintin anyway?

We readers of the series, think we know tintin very well. However, if you think about it, we actually don't. We don't know how old he is, for example. I used to think that throughout the series, he was in his twenties. I bet many of you agree he looks very young so any age above the early twenties is not likely to be his age. Luckily for us, Herge gave us an age range for Tintin through the series. Herge said that Tintin is thirteen in the first book and nineteen in the last book. Tintin is only thirteen in Tintin in the Land of the Soviets and only nineteen in Tintin and the Picaros! That's young. That means for most of the series, Tintin is a minor, living alone! From my calculations he is fourteen in the Cigars of the Pharoh arc. Also, I calculate he is sixteen from Crab with the Golden Claws to Red Rackham's Treasure. I guess our young reporter was younger than I thought.  How old do you think he is in books like Tintin in Tibet and The Calculus Affair?