When talking about work/life balance. As someone mentioned earlier, it really should be translated to quality of life. If someone is single, working 100 hours per week in a city they love and in a job they love, their quality of life is high. It's personal, likewise, someone who is very social or close to friends and family would value social time more.
I'm assuming that most people on here don't have children. For me, its a major consideration for quality of life.... It comes down to your personal preference... Do you want to spend more quality trime seeing your career/company grow or your children grow?
The main factor which influences my career choices was when my wife showed me some of the drawings the children in the classes she teaches made. They were asked to draw the most important people in their life... Most of children who had both parents working in IB or some occupation which meant they worked long hours drew a picture of their nannies/domestic helpers. Maybe for them, they don't really care, but that would be heartbreaking for me.
But definitely, once you have children, priorities need re-shuffling. Some people see their legacy as the influence they have on a company. I definitely see my legacy in terms of my genes.
Sure you could retire at 45-55 with a huge wad of cash, and then try to start a family... but it will be more difficult (unless youa re lucky), moreover, the later you start, the increasingly unfair it is on the children, imagine being the only child who had a pensioner for a parent when still at school?