I think it would be most rational to base voting rights on what's called the "age of majority" -- the age at which point one becomes a legal adult.
And it would be most rational to pass a Federation-wide law saying that the age of majority for any given species falls to that species's indigenous government to determine, with Federation and Federation Member governments being obliged to so recognize a given individual as reaching the age of majority according to the standards set forth by the indigenous government.
Makes sense to me. But I can see some further complications arising.
What if there is more than one government of a single species? Mars is supposed to be an independent Federation member world, at least in the novels. There are probably other independent human worlds in the Federation. Even if they are just colonies and not full member worlds, it's likely they would have the autonomy to set their own age of majority rules. What if they set different standards than United Earth? Which rules would Andor use for it's human citizens? Earth is the human homeworld, of course, but for all we know there could be more humans living away from Earth than on it.
That's a fair question. My suspicion would be that this issue simply doesn't come up that often, and that by the time a given species has spread across multiple worlds, it's probably already adopted a uniform age of majority.
Then there's the problem of whether it's right for the humans who were born on Andor and are Andorian citizens but are not United Earth citizens (and therefore can't vote for the Earth parliament) not to have any influence on when they reach their own age of majority.
Well, frankly, I don't see why it
would be right for someone to apply standards derived from one species' biology to another's.
Another complication, IMHO, for voting Age Rights would be, if a member world was a Geritocracy, who view the Wisdon age above all else. I can't really see that Society wanting folks barely into adulthood voting, before they had gained any Adult Wisdom.
I don't see why a society that practices age discrimination when it comes to voting rights would be allowed into the Federation in the first place, any more than a society that restricts the franchise to property owners, or to males, or to members of certain ethnic groups. It wouldn't be a democracy, so they shouldn't qualify for Federation Membership.