That's a fair point, but you can also flip it the other way: someone with 5 years left on Earth might be much more interested in improving things for future generations. Thinking about how your impact will live on after your death is an enduring human strategy for dealing with the terror of impending nonexistence. Another way to flip it: someone who's 40 has plenty of time left to live hedonistically without having to face really long-term consequences of their actions.

It's also worth noting that short-vs-long-term planning is just a paradigm for thinking about your politics; in practice, absolutely horrible things can arise from both strategies. Long-term thinking can manifest as clinging to tradition (since you now feel ownership over future generations and want to impose your own world-views on them). It can manifest as early optimization. Both of these things can be good (maintaining culinary/aesthetic culture, infrastructure investment) or bad (maintaining brutal and oppressive traditions, prematurely and hence inefficiently optimizing things at tremendous economic/social cost).

I'm not saying the status quo is anywhere near optimal. I think it's a very interesting (read: hard) and probably culturally-dependent question of what age groups are able to minimize the societal happiness cost function (and of what that cost function even is). It's incredibly difficult to say anything generally meaningful about how you might do this well. The one-person-one-vote system is at least a simple solution that seems acceptable to most, which is an important feature (not that each person's vote counts the same in practice in USA, but that's a whole other problem).