According to many recent articles, brains don’t finish developing until the late 20s. Some use this to justify not listening to (or even giving basic human rights to) those under 25 or 30. But how strong is that logic?

We all know life’s trajectory. Birth; childhood; puberty; adulthood; old age; death. What happens to brains along the way? At conception, when a human is just a single cell, we might well assign it an intelligence of zero. But inside the womb, the brain develops rapidly, getting smarter and smarter. After birth, a child gains all the normal abilities – walking, talking, writing – in about ten years, eventually reaching normal human intelligence. Growth then levels off, peaks, and eventually begins a slow decline from old age, reaching zero again at death.

To graph it out, it might look something like this:

(That’s real data – the different curves are ways of measuring intelligence.) Looking at the data, it seems to support the original theory – brains don’t reach peak potential until age 25 or 30, depending on how you measure.

But wait. The late 20s are the peak of the curve. Intelligence peaks in the late 20s – but then it starts going down, as neurons begin to decay. Sure, people younger than about 27 aren’t as smart (on average), but people older than 27 also aren’t as smart. If 27 is the peak, then (by definition) it’s worse before, and it’s worse after. If we don’t want 16-year-olds voting because their brains are underpowered, then we also shouldn’t have 60-year-olds voting, since their brains are equally underpowered.

If we’re going to restrict voting based on age – something most agree is a good idea – then it seems like the only sensible way to do it is defining some measure of mental ability, setting some level X that we think is high enough to vote, and barring people when they’re (statistically) so young or so old they’re below X. After all, if we don’t want teenagers to vote Britney Spears, why aren’t we equally afraid of octogenarians voting for Strom Thurmond?