That good hitters increase power as they age is something which Bill James proved back in the mid-1980s, based upon stats that preceded the steroid era -- in some cases by decades.

This shouldn't be surprising. Power comes at the expense of contact. It is no coincidence that the list of all-time home run leaders is virtually identical to the list of all-time strikeout leaders. A young player who strikes out a lot is less likely to make the majors unless he's already a home run hitter, so there is considerable pressure upon him to focus on contact over power. Ryan Howard is the exception, not the rule.

Once established in the majors, however, a player at or just past his peak years can easily see that increasing his home run rate will bring greater monetary rewards even if his strikeout rate increases somewhat. Thus, as players age, they begin to hit more home runs because it's more financially rewarding to do so, and a swing can be tweaked to increase the home run rate without major loss in other aspects of hitting.

Two other factors are also involved. First, young players are often not yet fully physically developed -- either in their musculature or the degree to which they are accustomed to the rigors of a full season. The latter affects even experienced players -- Daisuke Matsuzaka's season took a sharp downturn once he accumulated the number of innings he was used to pitching in the shorter Japanese seasons. Dusty Baker once recalled how Henry Aaron advised him to look over the schedule at the beginning of the season and pick out games which he would probably sit out, simply to rest himself. A younger player, of course, wants to be out there every day -- to show off his skills and establish his place. An older player has learned to pace himself.

Second, the running speed of a player normally decreases as he ages, forcing many to find new ways to compensate for the diminution of an important skill.

Finally, Moneyball noted that Giambi (and Bonds) were superior hitters not only because of their ability to hit for power, but because they were unusually PATIENT. They knew the strike zone better than anybody else, and had the discipline to refuse to swing at pitchers outside of it. Nobody, as far as I know, claims steroids help you learn how to lay off a curveball in the dirt.