It's exactly because of your point 1 (those with a stake in the future of the system) that I'm somewhat in favour of lowering the voting age, and making voting

It's exactly because of your point 1 (those with a stake in the future of the system) that I'm somewhat in favour of lowering the voting age, and making voting non- compulsory (I'm in Australia) for those over retirement age. I haven't observed any evidence that people vote more sensibly at 21 vs 18 at any rate. But there's strong evidence that those over 50 or 60 vote to preserve the status quo even when it's clearly not working well for all sorts of rational and less-than-rational reasons. And yes you did mention stability too, which is important in the long run obviously, but not always best maintained just by avoiding intentional radical change at all costs.