By that line of reasoning, we may as well eliminate tax brackets entirely. Or any sort of arbitrary limit, really. What makes an 18 year old mature enough to vote, but not a 17 year and 364 days old?
Humans, as all animals, have a built in maturity limit, which is puberty. After completed puberty you are biologically an adult. You can be a completely immature fool though biologically adult, as well as at 18, at 38 or until death. I'd say a majority of people live their whole lives without ever reaching adulthood in a spiritual sense. Go work in the service industry catering to an elderly clientele if you don't believe it.