it isn't a given that he wouldn't be against breaking the odd law here and there.
Good point. I think there is a bit of a contextual difference, though. Tintin frequently exhibits his abilities to think on his feet and adapt to almost any situation, often breaking the law in the process. To my knowledge, however, he only freely bends the rules when he is in some crisis situation.
There are numerous examples in the books where Tintin does what would appear to be many age-restricted actions... drinking (The Black Island), traveling alone (King Ottokar's Sceptre), presumably owning a firearm (The Broken Ear)... And these all appear to be undertaken without the influence of a crisis, or under duress, but rather perfectly at his own convenience and election.
To me the environment Herge created for Tintin would seem to imply (most of the time) that Tintin is around his early twenties, maybe even a bit older.
FormulaFourteen is looking at it in the context of the real world and not whether or not Herge checked up on local laws
Very true. It seems to me that Herge strove very hard to depict Tintin as credibly and realistically as possible, so it makes sense to me to analyze him as realistically as possible. Obviously, though, you can only take this line of reasoning so far before it begins to get far-fetched!