You can still be a dependent at 18 (and for many years after), so that's not really a good qualifier for voting rights.  

As far as development, the human brain typically doesn't reach full development until 25 (and some studies push it as far as 45).  That's not really any research targeted to the differences between say, a 17 year old and an 18 year old.  It mostly looks at bigger ranges, like 13-25.  

Regardless, we don't take into account mental development past 18.  Someone suffering from impaired development, or who suffers a loss of cognitive function as the result of a traumatic injury, is still permitted to vote once they turn 18.