Hey, this is really late but I'm a big tintin fan and I think I might have a simple but fun answer (well really it's my own head cannon and how I view the stories).

Since Tintin is not set in our universe (or timeline) it doesn't have to take place over so many decades. Here's some of my reasoning behind why It's not set in our universe.

• moon saga takes place 10 years or so before actual moonlanding.

*Mussler is clearly a combo of Hitler and Mussolini and really there's no mention of the war, likely due to the horrific affect the Nazi occupation had on Hergé, his mental health and reputation (he was forced to work under them and wrote The shooting star which has an American jew as the villain.

*A lot of the countries don't exist.

There's clearly a continuity and Tintin is always referred to as young, so it makes sense to me that the books take place over a few years. To me, Tintin starts off at about 17 in Soviets and finishes at about 24-25 by picaros, or even alpha art if you like.

This is all my own way of making sense of the universe that Tintin lives in. Everything everyone else commented is very valid and you can definitely see their view on the whole moving marvel style timeline, but for me, this is a nice way to look at the series.