If Megaton was really 200 years old, I'd suspect most of the walls would have deteriorated by now. Didn't she say something along the lines of the original founders getting the bits and pieces for the wall from a nearby airport?
The founders dragged aircraft peices from the local airport its remains now gone over time, to build the walls and some of the interior as you can see by a fuselage inside megaton. Although its offtopic i never saw the logic of why if they could drag aircraft wings and fuselage, why they didnt drag rail cars, or even pull up the existing rail lines, if they can build megaton, then they would be capable of that.
Also none of the wrecked buildings looked weathered, in the time since the bombs general deterioration from the ruin and loss of structural integrity plus weathering would affect them, also the stores wouldnt be in that condition if the bombs dropped around them they would be all burnt out, fire is a major factor in a nuclear blast.
But weather affects in games arent that great, i played wow when they introduced rain to the game because everyone wanted to see what it was like when it rained, other than the fact that it slowed the game up, it was actually pointless as it made no change to the game other than losing vision at distance, and you had the option to turn it off which i did, making the game run that little bit slower was more of a problem.