The young think they know everything because their life experience is so limited. As you age you (hopefully) begin to realise how truly enormous the wider world of experience is.
Your knowledge increases but the understanding how how much there is to know grows even faster.
Shit, the older I get the less I understand. Nothing makes sense anymore and I don’t know what I’m doing.
Alternate take:
The young think they know everything because their life experience is so limited. As you age you (hopefully) begin to realise how truly enormous the wider world of experience is.
Your knowledge increases but the understanding how how much there is to know grows even faster.
Mastery of a subject is understanding how little you actually know about that subject.