[superuser2]

Retirement is the state of discontinuing employment and living off one's investments/Social Security instead. What's to stop him is that if he continues to be productive towards the ends of some business he is by definition not retired.

[fredkbloggs]

There are modes of living other than just employment and idleness. Entrepreneurship is one, but even if you consider that employment by another name, it's not the only one.

[superuser2]

Yes, and those which involve working for money are not retirement.

[fredkbloggs]

But the essay, such as it is, was about usefulness or uselessness, not money. And in any case, there are hundreds of millions (more probably billions) of humans on this planet who neither have nor need money. Your concept of retirement is narrow and necessarily bound to a particular mode of living, as well as largely unrelated to whether someone is useful. The idea that the three phases of life are childhood (idleness), employment (saving money), and retirement (spending money) is both historically and culturally anomalous. It's more constructive to think about the many modes of living and their relationship to one's personal utility and sense of worth than to dwell on labels, especially since so many people live in ways that are hybrids or entirely outside the system your labels assume.