Hmm. An answer for that might be that whilst many planets might have many different species, the voting age would be based on one's species rather than on where one lived.
So even if a human decided to move to Rhaandar and become a citizen of that planet, under their laws and rules, seeing as it's unlikely they would ever reach 150 (what is the Rhaandarite age of adulthood), then they would be permitted to vote by whatever age humans were.
It would be a tricky thing to sort out, as species ventured out into space, joined the Federation and had new people deciding to settle on their world, as their legal system would need an overhaul and several new amendments. Though I think that if the voting age would be based on species, rather than planet.