There is a mode of adulthood that is dangerous and worth mentioning, which is a kind of hard-hearted acceptance that the world isn't fair, life is full of suffering, and there's nothing you (or anyone) can do about except suck it up and keep going. There's a kind of grim determination to continue despite not wanting to. Maybe it's because you have people depending on you, or maybe it's because you're just very stubborn. To some extent, I associate this mode with the opening scene of "There Will Be Blood" when Daniel Day Lewis' character is injured and spares not a single moment on despair, or self-pity, and gets himself out of a miserable situation, against the odds.
In this mode, there is little time or urge for compassion, or play, or joy. You are a cog in a large machine that values you only and precisely according to your economic function, who's body participates in multiple overlapping patterns of decay, and that's it. Maybe, if you're religious, you can look forward to a happy afterlife (and indeed one can imagine the role of religion in such a person's life is to offer a reason to not act out on the rage that inevitably underlies such an existence).
Almost everyone on HN is lucky to not have to endure this (or at least, for any length of time). And indeed, startups cannot succeed with that grimness. You must maintain your sense of play and joy, I think, to be successful in this space, because this is the most playful and joyful part of the economy! We make stuff that makes people go "ooh" and "ahh", and how can you really do that if you're an adult?