Here are life's transition moments:
1. Undergraduate to Graduate school
2. Graduate school to Postdoc(s)
3. Postdoc(s) to a "Real" Job
4. Early Scientist to Mid-Career Scientist
5. Mid to Late
6. Official "Retirement"
Somewhere in there, there is also a side-step transition into motherhood or fatherhood, and then back. Some may have more than one postdoc. Some may have many mid-career transitions. Some may have multiple careers such as an academic administrator, CEO, author, mentor.
One of my advisors retired recently, and I found that he was faced with a similiar career transition. He still wants to give back, but was always the professor and researcher. It may take him a while to find his niche after the "official" retirement. Having said that, he faces the same issues as we do, transitioning from postdoc to industry, motherhood back to career.
Career transition skills continue throughout life. It only really hits grad students and postdocs, as they are no longer told which class to take, which papers to write. Students and postdocs have to use their own career transition skills, combined with their science to come up with their career niche. That's challenging for some, as these skills have never really been fostered for them.
Going from school to postdocs and perhaps even the first job is the easy part. When the expected job progression is "modified" with children, aging parents, downsizing companies, outside interests, then life becomes interesting. Life brings forth an "intermission" time which you can reflect and self assess. These are the rare moments in life when you can see what you are really made out of.