Long-term success: the thriving of our species.
So we seek happiness. Not contentment, but happiness. We are sold ideals that represent happiness and feel unhappy when we don't hit them.
It's great when you find someone that is generally happy with health, stability and family. I suspect that's a minority in this space, where there are overnight millionaires, CS savants and business success everywhere you look. Some work their tails off for decades and never hit the mark. Some see fleeting success and then dead ends.
How does one transition from "I want to bust my ass, blow everyone away with my startup and walk away set for life" to "I have weekends off, a great family and a path to a comfortable retirement?" Are some people just wired like this or is it trained behavior?
It's probably a combination of the culture and expectations around the individual rather than the individual themselves. Children are pretty often told they can be whoever they want to be by society, and so their bar is set pretty high. But when was the last time a 30 year received the message that they could be anybody they wanted to? People expect different things of older people, and the whole situation, with all of the narratives we use to define ourselves and others, is probably pretty self reinforcing. The attitude also likely shifts as you have a family and they grow up - again I think it's about what people expect. No studies to cite here, but just throwing out some ideas.
I guess better questions are: why do you care if you're successful? How much success do you need?
Think about it: financial success? Power? Expression? It's easy to assume these sorts of things define success, but I think it's easy to come up with tricky exceptions.
We like to view life as a sequence of accomplishments, because it helps us understand others' lives. But does it make sense to frame your own life that way?