A big part of getting older for me is the death of enthusiasm. Am not nearly as excited at the thought of starting new things like I was a decade ago. It is something that I rarely think about, but when I do certainly feel somewhat saddened by the reality of present nature. It isn't something that can be changed at the snap of a figure either, as inorganic enthusiasm is as good as fake.
The energy is still there for whatever I want to chase, as am still quite young. However, having a series of varied results over the years means that there's have a realistic set of expectations whenever something is being pursued. One quick lesson life teaches is that we don't always get what we want, and when it becomes the majority of times, it is sort of ingrained in our memories that such is the nature of things.
I usually joke with myself that it can be depressing to be an adult sometimes, as the freewill to dream is overshadowed by a much more nuanced expectation of how things will ultimately play out. This manifests itself most in how much energy and effort we are willing to put into it over a sustained period of time. Even starting energy, which lights the fire and sets the tone is something that isn't quite as present as it was when we were younger.
Of course a nuanced expectation doesn't mean a dead one, and isn't always negative. It can always help us take more calculated steps towards the desired goal. It doesn't feel that way most of the times, but there's always the possibility of actually accomplishing the tasks we deem realistic.