It's been a frustrating season for the St. Louis Cardinals, from shaky bullpen management to injuries to continued, head-scratching moves from the manager.

But hey, at least they didn't sign Jason Heyward.

In the interests of full disclosure, I thought that they should sign Heyward. While I was always a little cool on his offensive production, his defense and baserunning consistently grade out as excellent, and most importantly, as I wrote last October, his age made him the most likely free agent on the market to sustain his value. I think I was wrong about that.

My mistake was to make the old assumption that aging curves are the same, that players improve until age 27 or 28 and then begin to decline. Of course, individual aging curves can show a lot of variation, because we are all special people with our own unique bodies. One provocative piece from Jeff Zimmerman a couple years ago even showed evidence that modern hitters don't improve - they come into the big leagues as good as they will be and then decline.

If that is the new model for aging curves (and there's certainly debate about that), Jason Heyward is Exhibit A.

Since integration, just 35 players have played a full season at or before age 20. Jason Heyward's rookie season at age 20 ranks 8th on that list in terms of OPS+. Ahead of him are six Hall of Famers, two very likely Hall of Famers (Mike Trout and Alex Rodriguez), and Tony Coligliaro, whose career was on a Hall-of-Fame trajectory until he was tragically beaned in the head.

And yet since then, Jason Heyward has never bested that 131 OPS+ of his rookie season. Of all those players to play a full season at age 20, he is one of only three never to improve on his rookie season. (Hat tip to Sam Miller of the Effectively Wild podcast for pointing that out on a recent episode.)

More than halfway into the first season of his 8-year, $180 million deal with the Cubs, Heyward's OPS+ sits at an all-time low of 78, or 80 on the wRC+ scale. During no month this season has it poked above a league-average 100.

Now, is it possible that Heyward just "started slow" and will rebound to at least league average as a hitter? I suppose it's possible, but we're having this conversation on July 20, not May 1. And while Heyward has never been a bad hitter, the trend-line is clearly going down. As Zimmerman's models suggested, Heyward looks like he came into the league as good as he was ever going to be, and continues to decline, even if he's only 26.

Physically - and completely anecdotally and subjectively - Heyward seems to match this profile. When you see old pictures of him, he looks like he was a full-grown man by about the age of 12. When he made his big league debut at 20, he certainly wasn't a guy who scouts talked about needing to develop more physically.

All of this is just a way of saying yes, Jason Heyward is 26, but he is an old 26. Those of us who envisioned several years of sustained peak performance, or maybe even a bit more development, were mistaken. The Cubs bought eight years of Jason Heyward's decline phase.

Now, in addition to the caveat that Heyward could still rebound a bit from the hole he's in now, he is still on track to be a 2.6 fWAR player this season by virtue of his still-elite defense and baserunning. That's still a "solid starter," but hardly the production you would want in the first year of an 8-year-contract for a guy you shelled out $180 million for.

And maybe it's just me, but I don't put quite as much stock in value estimates on baserunning and defense, which seem more variable. At the very least, I'm glad I don't have to convince myself the Cardinals were right to sign a below-league-average hitter at a corner position to a long-term deal because he runs the bases well and plays good defense.

That's not to say the Cardinals in any way deserve to be commended for not signing Heyward. Though I don't think we've ever heard exactly what the Cardinals offer was, it was reported to be in excess of $200 million, though with a perhaps different opt-out structure and a lower average annual value. It seems the only reason he didn't sign with St. Louis was some combination of liking the Cubs package better, wanting to be there if they win a championship, and believing the Cardinals were all old men who listened to Dad Rock.

Baseball is Weird™, and in this instance, the weirdness seems to have worked out in the Cardinals favor. Last offseason, Heyward looked like the cornerstone player the team desperately needed. When he didn't sign, it looked like Piscotty and Moss would desperately have to run back-and-forth between first base and right field to scratch any value out of those positions.

Instead, Piscotty has become a fixture in right field, the team's 2nd most valuable hitter while also ranking 2nd among NL right fielders in defensive runs saved (behind Heyward). The team, which didn't get their top target on the offensive side of the ball, ranks first in the NL in wRC+ and third in home runs.

A lot of things have gone wrong for the Cardinals this season, but not signing Jason Heyward hasn't been one of them.