To me, it's when you pay everything off, have fulfilled your obligations (kids, etc.), and then have enough rolling in to pay for the lifestyle you are willing to live within.
I'm about 40 and while some pieces I wrote discuss leaving the federal government around 2030 to lock in the pension I can collect in the year 2042 at age 60, my youngest daughter is 10. If it's looking like she (or the other 3) are heading to something like med school, I'm going to hang in the workforce a bit longer. After all, I'll have the time and will be making $150,000+. If I get an offer to teach full-time, then I'll "retire" into teaching, which I anticipate doing until about age 60 when I can begin collecting both my federal and state pensions.