I honestly didn't mean it to be a crack. It's just that from what I've heard... PI tends to have less stress and more freedom - it'd fit somebody who already has their retirement set but still want something to do routinely.
But hey, if people want to go into PI from the get go, then all the more power.
It depends on the PI setting. I've seen a more relaxed office and an office in which people easily work as hard and as many hours as they would in a firm, at a fraction of the pay. Some places probably do offer more in the way of work/life balance, but PI's not uniformly or even usually less work or less stress than private practice. Actually, the work may be much more stressful because of interactions with huge bureaucracies like welfare and public housing agencies, clients who endure a great many external stressors that interfere with their ability to participate in litigating their cases, and emotionally difficult subject matter. I work with low-income domestic violence victims. It takes a bit more than just wanting something to do while cashing in a retirement account to draw up a petition for an order of protection detailing how somebody's husband raped her with their two year old on the bed.
But joking about my cane aside, I (am not that old and) have been PI-oriented since I was in undergrad; law is just another approach to a career-long theme, and I would say the other people in the public interest program at my school are probably a little older than the average student but not really middle-aged or second-careerists (unless that first career was also in something social justice-related). The PI lawyers I've known have included a mix of people who've come from private practice and career-long public interest attorneys. Some seem to have been looking for a change of pace from a previous corporate-related career, but the latter's more common than you're implying here.