That's all because when you are little or just very young (an adolescent), when you behave, think and speak like an adult (or at least, in case of children, to this extent it's possible for a child, for a teen is basically like a normal adult, just a bit younger and the fact now teens are immature has its source in social expectations - throughout centuries teens were treated like adults so they behaved like ones and had duties of those - it wasn't taken from nowhere - a teen has the full potential of adulthood), that's already something with a capital s. But when you actually achieve adulthood, that's normal for you then to speak, behave, think etc. like one, so no one is going to be impressed any more. And when you are an adult person, you are expected from more than when you were a kid/teen, in terms of your level of maturity, self-independence and life experience. Only this time, unlike it was when you were very young, you can't fool yourself you are ever going to get normal, be like others.