One of the surest signs of middle age is that you actually listen when outsiders tell you that maybe it's time that you started to slow down. Consider, if you will, what's happened lately with Microsoft, Amazon. com and Wal-Mart, all of which were once treated by Wall Street as high-growth companies. The more money they spent upgrading facilities and expanding into new markets, the more Wall Street loved them. All three had revolutionized their industries, were growing like mad and were more about tomorrow's potential payoff than about today.

Well, today has arrives. The Street has issued a collective judgment on our three amigos—it's declared them to be middle-aged. It hasn't done this formally, of course. But if you look at how the Street has treated these three stocks lately, it's the only conclusion that you can draw.

Being considered less-than-youthful isn't a total shock to Microsoft, which showed signs of middle-age onset when it started paying serious cash dividends a few years ago. But it's surprising to see Amazon and Wal-Mart act middle-aged. They both had seemed to be expanding without end but they've now decided it's time to slow their growth, at least in part to help keep Wall Street happy. Middle age, you see, has nothing to do with how old a company is—it has to do with how it thinks.