I imagine that from the birth of self-awareness we've been trying to fight our own mortality. We're the first species to know its place on the Earth and comprehend the brief flicker of our existence. It must have driven some early humans crazy.
When I was younger I didn't realise how quickly ageing happens. When you're a child it feels like life is infinite. You learn better, but you don't accept it. It creeps up on you before you can notice. There's no dramatic shift in your thoughts or personality, only how the world sees you. I thought adulthood changed you forever. You'd become a mature, responsible person with life all figured out.
As I got older I saw the reflections of "childish" qualities in adults around me. They didn't have it figured out, they were like me only with less time in the bank. They were still fundamentally the same people they'd always been. The likes, dislikes, personality, it all stays intact despite the ageing. The truth is we're forever young, we're just trapped in our decaying bodies.
In my early twenties, I waited for that moment where I'd feel like an adult. Now I know it'll never come. Life is just a long process of growing and accepting the shift in how you're perceived. Before you know it, you're considered "adult" and you're not sure what it means. Maybe it's when you accept the inevitability of ageing and don't let yourself drown in the perceptions. I am what I am, the labels don't matter.