When I was a child I longed to be an adult. This longing was based upon two presumptions: first, that adults were free to do what they wanted to do (within the limits of law and morality, of course), and, second, that they knew exactly how to do these things (i.e., that they acted with complete self-assurance). I have since learned (speaking solely for myself) that those two presumptions were a bit optimistic.
The following poem by Frances Cornford (1886-1960) examines this state of affairs from a different angle, but it perhaps expresses part of what I am trying to get at.
I used to think that grown-up people chose
To have stiff backs and wrinkles round their nose,
And veins like small fat snakes on either hand,
On purpose to be grand.
Till through the banisters I watched one day
My great-aunt Etty's friend who was going away,
And how her onyx beads had come unstrung.
I saw her grope to find them as they rolled;
And then I knew that she was helplessly old,
As I was helplessly young.
Frances Cornford, Collected Poems (1954).