This is the number we've been using recently, but it's been different in the past and there's no reason to keep using it, especially when we have data showing that 25 is the age your decision making faculties are generally fully developed
When I was a kid the argument was “if you can be drafted and sent to war you should be able to vote”. The state is still going to want those young able bodies for their militaries. Raising our age of adulthood to 25 seems unlikely to go over very well.