By Joe Posnanski

After a one-week vacation, we are back with the continuing evolution of an experiment that last appeared two weeks ago: a combination column with Boston Red Sox senior advisor and baseball writer extraordinaire Bill James ...

Today's topic is actually an age -- 33 years old. Many years ago, Stan Musial set a baseball player's prime from age 28 to 32. And even though this isn't 100 percent true*, there is truth in it. For many good-to-great players, 33 is the age when they begin to grow old. Maybe the bat slows a touch. Maybe nagging injuries nag more. Maybe the legs lose a little bit of their spring. Maybe the shoulder aches when they try to throw home.

*Bill famously debunked that prime years ago; he showed that a player's prime is quite a bit younger than that -- roughly from age 26 to 30. He says the numbers has moved some through the years, but the descent certainly begins before 32.

Whatever changes, 33 is an age when many players find that they can no longer do the things they once did. Right off, we should say: This isn't true of all players and not even most players (and we are talking every-day players here, not pitchers). Bill figures that about 70 percent of players perform about the same at age 33 as they did at age 32.

But, he also figures that more players -- and especially more GREAT players -- find 33 to be their most punishing season, the year that long fly balls stop leaving the park, the year that groundballs stop rolling through the infield, the year the bat feels heavy in July and August.

This is true this year, just like it is true every year: Alex Rodriguez, of course, is 33 years old and he in struggling in many ways. David Ortiz is 33 years old and he is struggling in just about every way (though he has been coming on the last couple of weeks). Alfonso Soriano, Placido Polanco, Edgar Renteria and Eric Byrnes are all 33 years old and all are having difficult years for one reason or another. Lance Berkman's batting average is way down. Carlos Guillen has been hurt all year. And so on.

Again, this isn't universal. Torii Hunter is 33 and he's off to the best start of his career. Russell Branyan is finally getting a chance to play in Seattle and at 33 he's been phenomenal. Bodies do age differently. And we are not even going to get into the whole discussion of performance enhancers ...

The point here is only that if you look throughout baseball history, 33 does seem to be the tough year, the one that players have to overcome.

* * *

Joe: Let's start with Royals outfielder Jose Guillen. I've spent much of this year watching him; Guillen has never been a GREAT player, but he has been a good player, in large part I think because of an unusually quick bat. In 2007 he hit .290/.353/.460. In 2008 he had a mostly lousy year, but he had about a five- or six-week stretch where he hit the ball about as hard as anyone I've ever seen -- he hit .390 and slugged .662 from May 7 through June 17, and many of his outs were smashes.

Well, he's 33 years old this year, and he seems in better shape, he seems more focused, he seems more determined than ever not to be a distraction for the team. But, again, he's 33. And you can see changes: His bat no longer seems as quick. This shows up in different ways ... he seems to be behind the fastball. He's seems to be taking more pitches. He seems to struggle against those third and fourth starters he once loved facing.

And it has been fascinating to watch -- I've never been a huge Jose Guillen fan by any means, but this year I have to admit that I've become a fan because it feels like I'm watching a player fighting with mortality. I see him, with men on base, bloop balls to right field rather than try to pull the long ball over the wall. I see him more willing to walk -- Guillen has been a famous hacker through the years, walking once every 21 or so plate appearances. This year he has walked 21 times in 240 plate appearances, which isn't exactly Barry Bonds, but it seems to be a shift in the way he plays the game.

Guillen's descent as a player really began last year, but this year, at 33, you can see it so much more clearly -- he can barely move in the outfield, he can't pull the ball hard except when a pitcher hangs a breaking ball, and so on. He has always been what the scouts call a mistake hitter, but more and more he finds that he's missing mistakes. Every day you can see how hard he's trying to adjust, though, and it's affecting in a way -- watching a ballplayer try to fight against time.

Bill: Historically, hitters' bats die at age 33 ... not always, of course, but there is quite significantly more loss in batting ability at age 33 than at any other age. Let me give you a few for-instances from history ... and obviously, I'm just hitting a few highlights; there are many others involving players with less recognizable names.

1) Hall of Famer Hack Wilson 1932, age 32: .297, 23 homers, 123 RBIs 1933, age 33: .267, 9 homers, 54 RBIs

2) Hall of Famer Al Simmons 1934, age 32: .344, 18 homers, 104 RBIs 1935, age 33: .267, 16 homers, 79 RBIs

3) Hall of Famer Heinie Manush 1934, age 32: .349, 11 homers, 89 RBIs 1935, age 33: .273, 4 homers, 56 RBIs

4) Hall of Famer Tony Lazzeri 1936, age 32: .287, 14 homers, 109 RBIs 1937, age 33: .244, 14 homers, 70 RBIs