If you go by post count alone, it'd be tough to judge. The old guard/founding fathers and mothers, are well repped as you would assume.
New school makes a good showing though. Clearly the boom occurred somewhere between 06-09. I think 07 and 09 classes are both far more active than any other class, but I'd bet that a ton of lurking takes place from earlier classes.
You have to set a criteria and it needs to be clear enough that you can distinguish each class from each other.