Invalidating a method because its "Dated" I was recently arguing with a college about a concept in economics and was told that my approach is invalid because it based on theory/method developed from the 1980 and that current methods don't approach the problem the same way. 

I cited my sources from a credible source which was published (NBER to be specific) however to him it was a good enough of a counter argument to invalidate my point. He did cite contemporary research which approaches the problem differently however he didn't address the sources I brought.

This isn't the first time I've heard the argument from my peers and I'm wondering if the "in with the new out with the old approach" is a general approach taken in academia.

Is the fact that a certain paper or theory is old or dated enough to invalidate it?
Older methods can be tied into older biases, mistakes, and bad assumptions.  In mathematics this is very rarely a problem, but in social psychology it can be a massive one. There may be bad mathematics involved, poor controls, cultural biases, confounding variables that weren't known of then, etc. I don't know where economics and rational expectations falls, but I'd hazard a guess that it's closer to the social psychology end of the spectrum than it is the mathematics end. There's a huge number of factors that impact economic behaviors, and any model is going to have to make simplifying assumptions to try to single out what the model's creators have deemed the most important and interesting things.

While research trends are not an objective evaluation of merit, it does tend to suggest that if something is no longer receiving attention then it is not suitable for addressing modern problems and concerns.  The exact reasons may be hard to pinpoint, but in this case may boil down to "today we're really interested in modeling how widgets and wodgets interact" versus "this older theory is meant to model how gadgets and gidgets interact".  It's not necessarily that the older theory fails to achieve meaningful insights on gadgets and gidgets (though it might, because some of its assumptions may no longer be considered good ones), but that nobody cares about that any more, and it is intrinsically unable to address what people do care about.