An 18-year-old once told me that the brain does not fully mature until age 24. That those younger than 24 do not have full cognitive capacity and cannot make fully rational decisions.

"One of the areas of the brain that matures late is the prefrontal cortex the area important in impulse control risk-taking behavior and judgment", says this site (the commas are all missing in the original, too).

Well, ironically, she was only 18 herself, so... should I have believed her? Or should I have shrugged off her statement as something any 18-year-old might say due to their lack in cognitive capacity?