I would imagine that the younger culms will have a thin sidewall, and the oldest will have a very thick one. I think you'll find that if you just cut a ton of them and bundle them and dry them out, you can spine and weigh them and get a reasonable idea which are older and which are younger. I would imagine the younger ones will end up very very low spine, and the problem will just work itself out.
With that said I have almost zero experience and I'm just goign on intuition here. Take my advice at your own peril.