This is a good question and there is a good scientific answer. I downloaded this for your information.
Brain Changes in Young Adulthood
At the same time that young adults are experiencing new levels of sophistication in thinking and emotional regulation, their brains are undergoing changes in precisely the areas associated with these functions. While it is not possible to determine cause-and-effect, brain and behavior are changing in parallel.
Prefrontal cortex: The most widely studied changes in young adulthood are in the prefrontal cortex, the area behind the forehead associated with planning, problem-solving, and related tasks. At least two things affect the efficiency in its functioning:
myelination: the nerve fibers are more extensively covered with myelin, a substance that insulates them so that signals can be transmitted more efficiently, and
synaptic pruning: the “briar patch” of connections resulting from nerve growth are pruned back, allowing the remaining ones to transmit signals more efficiently.
Connections among regions: At the same time, the prefrontal cortex communicates more fully and effectively with other parts of the brain, including those that are particularly associated with emotion and impulses, so that all areas of the brain can be better involved in planning and problem-solving.
“Executive suite”: The cluster of functions that center in the prefrontal cortex is sometimes called the “executive suite,” including calibration of risk and reward, problem-solving, prioritizing, thinking ahead, self-evaluation, long-term planning, and regulation of emotion. (See Merlin Donald, Daniel Keating, and others in References.) It is not that these tasks cannot be done before young adulthood, but rather that it takes less effort, and hence is more likely to happen.
20s and beyond
According to recent findings, the human brain does not reach full maturity until at least the mid-20s. (See J. Giedd in References.) The specific changes that follow young adulthood are not yet well studied, but it is known that they involve increased myelination and continued adding and pruning of neurons. As a number of researchers have put it, “the rental car companies have it right.” The brain isn’t fully mature at 16, when we are allowed to drive, or at 18, when we are allowed to vote, or at 21, when we are allowed to drink, but closer to 25, when we are allowed to rent a car.
I think this supports the advice to wait.