Children don't understand subtlety, or semantics. They don't understand that the things they are told by adults aren't absolute or always literal. Children have no innate concept of nuance. If you tell a child repeatedly that they can be anything they want to be, if they trust you and believe you, then as adults, those children will wonder repeatedly what it is that they keep doing wrong, why they are repeatedly disappointed, and why in spite of all efforts and good intentions, they are not always able to be who or what they want to be.
I think that the problem with the phrase "You'll understand when you're older." is that once we are older, and we do understand, we have built so many habits around the things we didn't understand during our formative years, that it becomes a major undertaking to change the habits, and far easier to give in to a sense of despair of futility, or addictions.
Should I believe that there are things I want to achieve, roles I want to fulfill that I simply will not ever be able to? No, that seems like a bad situation... and yet it seems necessary to be able to accept that there may be things I can't achieve or roles I won't fulfill in order to avoid running my head into the same walls repeatedly.
I have wondered, since I was a Junior in high school, how it is that we teach our children the things they need to know to be happy, healthy adults, without steering them to build their foundations on the same mistakes that we have made, or have had passed down to us. More than financial considerations, or all the things I'd like to do before I'm "tied down with a family", the thing that makes me pause to wonder if I would be ready to raise children, is the question of whether or not I am capable and prepared to teach them how to be happy and healthy human beings as children and as adults later.
I cannot be whatever I want to be. I have not learned how to stop fighting a losing battle before I start to bleed out. I have fought with depression and shame and issues of self worth for 30 years, and only now begin to see how the things I was taught with the best of intentions as a child, were misunderstood by a child who accepts knowledge innocently as literal. So, where do I go from here? How do I learn to be happy while accepting all the things I want but cannot have or be? How can I feel strong and worthy of respect after giving up on dreams or aspirations?
I guess, in the end, my dream, the big one, is to be happy, and that's the one I can't give up on, so if the repeated failures, the bleeding out from fighting losing battles, is keeping me from my dreams, then I have to give up on small things to improve my odds of getting to the big ones... Small comfort, but I may still learn, and make it into a new and healthy habit.
For now though, it hurts a hell of a lot to want to be something I cannot be simply by my own sheer will and effort.