Wow i don't know why but i always thought Tintin would be 18-20 years old due to the fact that every now and then he drinks,
"The Broken Ear" & "Seven Crystal Balls" come to mind.
That's true, but I don't know what the law was in 1930s Belgium. Maybe there wasn't a legal age limit for alcohol consumption, and even if there was, maybe it wasn't strictly policed.
He also would be one very mature 15 year old.
Indeed, but I think that unrealistic maturity was deliberately very much part of the appeal of Tintin, right from the start: the fantasy for child readers of being only 14 or 15 but having none of the restrictions of real youth (parental discipline, school or a lowly apprenticeship, lack of maturity, limits to your driving skills or finances, etc). From all accounts, Hergé disliked much of his childhood so was probably living out his own fantasy youth through Tintin - exaggerating the sort of freedom he did find in the Scouts - as well as providing that fantasy for his readers. A much more realistic Belgian childhood can be seen in his Quick and Flupke strips.
like when he tells Lt. Delcourt in Crab that he never drinks
He actually tells him he never drinks spirits
. In the same book, Tintin accepts a beer from the Thompsons, and is happily planning to survive on champagne when barricaded into the hold cabin on the Karaboudjan, telling Snowy something like "Well at least we won't go thirsty!" So I think the book makes a distinction between Tintin's consumption of less-strong alcoholic drinks in a social or emergency context, and Haddock's uncontrolled addiction to the hard stuff.