Are younger people less capable of driving? I've seen articles saying that teen drivers have the most accidents out of every age group. Those articles also claim that teen drivers are less capable of driving as their brains are less developed than someone 25 years of age or older.

Are these claims that younger people are less capable drivers true?
Research shows that teenagers are not any less capable at driving, per se, but that the adolescent brain undergoes a period of neural changes that often lends itself to risk-seeking behavior. From Somerville et. al (2010):


  In adolescence, there is a heightened propensity to engage in risky behaviors that can lead to negative outcomes, including substance abuse, unprotected sex, inflicting harm on others, injuries, and death. According to the 2007 Youth Risk Behavior Survey (YRBS, Eaton, et al., 2008) the four leading causes of death that account for 72% of adolescent mortality – motor vehicle accidents, unintentional injuries, homicide, and suicide - are preventable. Such statistics suggest that these fatalities may be attributed, in part, to poor choices or risky actions (e.g., accidents, injuries) and/or heightened emotionality (e.g., suicide) underscoring the importance of understanding the biological basis of emotional and incentive-seeking behavior of adolescents, the focus of the present review.


The paper goes on to discuss how the intensity of young adults' emotional states relative to those of adults, as well as a heightened sensitivity toward reward (as well as an immature 'self-regulatory competence') are all underlying factors.

I think that this is what your article is referring to when it mentions that teenagers have 'less-developed brains'. On the other hand, one could argue the opposite. With puberty and adolescent development comes an influx of hormonal changes. With these changes come developments and mental states that are novel, new, and (at first) difficult to control. From this perspective, it is less a question of the brain being 'less-developed' -- on the contrary, there is 'too much' new development in puberty and adolescence, which can be overwhelming. As we learn to cope with these new developments with time and experience, they become easier to manage and control.