Tintin in Tibet: Part 1 - S2-E6
Revealing mistake: The shot after Tintin says "We can't give up now" shows Tintin, Haddock and the Sherpas hiking through a dry hill. Over the first hill is a part of the ground that is black. As the shot continues, an extra part of black ground instantly appears from nowhere, as if the animators forgot to draw the black ground during the first half of shot. (00:11:55)
Answer: According to Wikipedia, he is described as being a youth anywhere from 14 - 19 years old. There is no specific age.
raywest ★