Totally agree on the utility of shorter life-spans for dealing with climate change (though it has the horrifying ring of a "final solution").

"The "wisdom" of the elders will become more and more out of touch with reality as change increases."

That's just an assumption, and probably a mistaken one. The current pattern of rapid change may be largely reducible to noise in hindsight, as the most valuable knowledge of today is made obsolete by the knowledge of tomorrow. So the patterns of change most relevant to human life may just as well be longer, not shorter, going forward. Or they may just be unchanged (which seems most likely, somehow). What's worse is that they'll be increasingly obscured by the "noise" of rapid change, making them more difficult to perceive (at least for humans). So euthanizing the over-30s might not be the best approach.