Speaking as a migraine sufferer, the "energy-level economy" is something I'm all too familiar with. It is true that when you have many demands you have to balance them against your resources and prioritise in a thoroughly unromantic and funless way, and it's also true that "growing up" in the financial and material sense seems to be about acquiring an increasing number of responsibilities that make just such demands on your time and mental energy. I've wound up off sick from work long-term three times in my life now, and it startles me every time it happens how much more creative energy I suddenly have, and how much more inspiration I get to do things like write and take up new things to learn.

As for youthful idealism, I think there's an element of the Silver Horde nature to its apparent evaporation too - your older self doesn't put energy into so many random things because you're experienced enough to make a canny guess as to which avenues are actually useful in the end. I'm sure childish open-mindedness does lead to wonderful discoveries sometimes, but I think what tends to get forgotten is that it also leads to a lot of wasted time in between them...