Why does it take so long for us?
In: 3
Because we’re smart! And because we walk upright.
So somewhere down the evolutionary line that led to us (big smart apes), it was determined that walking upright 100% of the time is pretty neat. We spend less energy moving around, and we can have very specialized hands that are good for holding tools (but very bad for walking on), we can travel long distances, etc. Walking upright, however, requires very narrow hips compared to walking on all fours. It also requires some very strong booty and thigh muscles to keep us standing. This is problem one.
Problem two comes from our big brains, especially the big thinking part (the frontal lobe). We need HUGE heads to accommodate all them brains. You can see how big our heads are compared to our bodies, specially our hips!
So, small hips+big heads=trouble birthing. To get around this, we evolved to give birth basically as soon as our babies are ready to survive the atmosphere (can breathe and swallow by themselves), while still being small enough to pass through their mothers. That means we come out of the womb woefully undercooked, and need to grow up outside the womb to adulthood. Growing up outside the nice, cozy, nutrient rich environment of the placenta also takes some extra time, especially for these big brains we keep talking about to develop.
There have been studies suggesting our overall hip/head ratio has shrunken a bit since cesarean births have become widely available, because our hips would no longer become limiting factors to the size of our babies.
Larger creatures take longer to mature. Elefants take 10-14 years to sexual maturity, which is kinda same as humans: https://www.britannica.com/animal/elephant-mammal/Reproduction-and-life-cycle
Of course, elephants are a lot larger than humans, but humans have couple other factors slowing their growth, as u/acdgf explained in his answer.
Animals needed to mature quick in order to survive and procreate. It was long process, lots of species didn’t adapt quick enough and gone extinct.
We evolved from animals which were relatively safe on trees (not many predators climbed trees and ones that did could be easily fended off with sticks to the face) and that ground walking fad is fairly new to our kind. So we as a specie didn’t need to mature quickly, we took our time in our safety in trees. Then we started walking on ground solely, no more trees. The problem of long maturing of the young persisted.
So we adapted by staying in larger groups to shield the young until they are big enough to contribute to the group. Didn’t always work because of big predators, but we traveled a lot and could hunt and scavenge in places other predators couldn’t.
Our brains continue to develop well into our twenties. So even though our bodies are done growing, our brains are still growing and maturing. That’s why it takes us longer to reach adulthood than other animals.
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