I suppose that, given his agility, Snowy is around one to six years old, about the equivalent of a late teenager to an adult.
In the black-and-white editions of Soviets and Congo, he's like a quite vain teenager, obsessed with how they look. From America - Island, he seems to have grown a few human-years older, but his nose is in good condition, his character is starting to show, and he's running at a good pace.
From Sceptre, his lust for food (and alchohol), as well as a great appetite, has begun - bad news for poor old Tintin! He's agile as ever as well, so by then, I would say he's a strong, plucky two-year-old. From Crab up to Crystal, he is mostly the same.
From Prisoners to Explorers, I think he's a good three-and-three-quarters-year-old. He's getting more cynical, independent ("Who do you think I am - granny's little lap-dog?" from Explorers) and is more mature.
Between Calculus to Tibet, he's more willing to make friends, more loyal, and is about as mature as he can get. I would conclude he's four, by now, and the last time he starts searching for food is when he finds the bones in the Migou's ice-cave in Tibet.
Castafiore to 714 has only slight differences in his character, but I think he's only just reached his fifth birthday.
Flash forward to the more modern Picaros and Alph-Art, and there's something that tells me that not only has Tintin grown, Snowy's gained another year as well, but thanks to a good diet and plenty of hiking, he's still as alert as he was six years ago.
I speak as an owner of a mischievous 3-year-old Norfolk Terrier! | 
