I get the impression from the question that you're after addresses with an established history (n months old) and not necessarily a history with your own contract.
This is probably not impossible to fish out of the blockchain using assembly but it certainly wouldn't be easy unless someone has a nifty trick to discover the block.number of the first transaction ever signed by
msg.sender. I don't.
I'm not sure it's a great idea. Shouldn't the goal be to limit voting to real constituents (actual players?) in proportion to their stake? More like a whitelist or one token, one vote?
Have you considered quadratic voting? Maybe there should be a small refundable deposit held in escrow to increase the temporary cost of participation. That would increase the cost of casting large numbers of votes (as well as gas cost).
Hope it helps.