Democracy

Just 200 years ago, England and Wales had a completely baroque electoral system, where in one place the Member might be elected by more or less anybody who had an opinion (conventionally not by women but it wasn't technically illegal, records do not indicate any women _trying_ to vote under this system) and in others only by members of a group actually _paid_ by the current Member (which is scarcely any sort of democracy at all), each district could have its own rules and most of them did.

Great Reform shook that up, introducing a baseline that would allow a typical middle aged skilled worker or professional of the era to meet requirements to register as a voter -- but it didn't result in a great upheaval, and over the rest of that century if anything the aristocracy actually expanded their control, because it turns out rich merchants can buy 100 votes in a district where only 150 people can vote anyway, but greatly expanding the franchise makes this game unaffordable for them - whereas if people vote for you, Sir Whatever just because they always voted for you, or your father (also Sir Whatever) before you, that's not difficult to maintain when more of them qualify to vote.

100 years ago, a lot of women could vote, though far more men (all men over 21), the introduction of widespread voting by women was seen as potentially very disruptive. In fact, the women proved no more spirited than the men, and more or less the same people (overwhelmingly men) were elected. Even when women were given the exact same rights (all women over 21 too) it made no major difference.

In the middle of the 20th century the UK got around to abolishing plural voting (yes, right up until 1948 "one man, one vote" literally wasn't the rule although plural voting had been somewhat neutered after Great Reform) and only in 1969 did they lower the age to 18. The newly enfranchised teenagers did not in fact tear down grown-up government, and things still continued more or less as before.

Among obvious groups that still ought to be enfranchised: Convicted criminals from prison -- at least all those convicted of crimes unrelated to the functioning of democracy, and frankly probably those too unless you're bad at prisons and can't keep them from tampering with the vote from inside a prison; Children -- certainly all teenagers and there's no particular reason not to enfranchise any child that seems to actually have a preference, their preferences certainly can't be less _informed_ than those of adult voters so why not?

Overall, given that the only function of democracy is to avoid the alternative (violent transitions of power) why shouldn't we let toddlers help if they want to?