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.