Worth noting that hobbits canonically get to be 90 to 100 years old on average, despite living in a society with what appears to be rennaissance or early modern-level technology. So they just have more time to figure themselves out than we do.
But consider this: a 50yo dude has more experience than a 20yo dude, but would he actually get that experience if everyone treated them like 15yo right up until they’re like 30? And in some ways, even today people aren’t considered fully grown up at 20 - e.g. many political offices have age requirements of 30 or 40 years (depending on where you live, of course).