The typical age for brain development for learning algebra -- a form of logical thinking -- is about the age of 14 in boys. This is more comparable to the skills used in voting than in handling a weapon.
Driving involves both types of thinking, and certainly young people -- especially boys -- do exhibit more risk taking than adults behind the wheel.
And yes, the argument has been made that adolescents shouldn't be as culpable for their actions as adults.
For example:
Brain development, culpability, and the death penalty
http://72.14.253.104/search?q=cache:l4n2bGqh60YJ:www.internationaljusticeproject.org/pdfs/juvBrainDev.pdf+%22adolescent+brain%22+%2Bgun&hl=en&ct=clnk&cd=2&gl=usFor many years, science had assumed that the adolescent brain was fully developed by the age of 14. It was thought that developmental changes in the brain occurred in the first few “formative” years of life. However recent scientific advancements indicate that the adolescent brain undergoes rapid change and does not fully develop adult capacity until the early twenties. 1 It should be emphasized that this development and change occur in all adolescents.
SNIP
It is clear therefore, that the normal adolescent brain is far from mature or operating at full adult capacity. The physiological structure of the adolescent brain is similar therefore to the manifestation of mental disability within an adult brain. These are not however the sole developments within the adolescent brain. It has further been found that cable of nerves (the corpus callosum) that connects the two sides of the brain appears to grow and change significantly through adolescence. 11 This cable of nerves is involved further in creativity and problem solving. The lack of a properly formed prefrontal cortex and corpus callosum indicates an impairment of the rational decision and thought making process instead placing heavy reliance upon the emotional and gut response area (amygdala). The ability to regulate emotions is therefore impaired and this can result in quite severe acts with little regard for the consequences. As Daniel Weinberger concludes “I doubt that most school shooters intended to kill, in the adult sense of permanently ending a life and paying the consequences for the rest of their lives. Such intention would require a mature prefrontal cortex, which could anticipate the future and rationally appreciate cause and effect. The often reported lack of apparent remorse illustrates how unreal the reality is to these teenagers. Adolescents need people or institutions to prevent them from being in a potentially deadly situation where an immature brain is left to its own devices. If a gun is put in the control of the prefrontal cortex of a hurt and vengeful 15 year old, and it is pointed at a human target, it will very likely go off.” 12
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The same argument that would exempt adolescent killers from the death penalty would support raising the age minimum on gun possession. As is stated above, the brain doesn’t fully form until the early twenties. Even an 18 or 19 year old has impaired function as compared to an adult with a fully formed brain.
Under the controlled situation of a military force, an adolescent may be turned into an effective soldier. But just buying a gun doesn’t turn an 18 year old into someone who can be trusted to use it wisely.
P.S. I have to add that I disagree with your idea that voting or driving is "much more serious to the world at large." A loaded gun in the hands of someone with less than a fully developed prefrontal cortex is a very serious situation.