You learn something new every year playing fantasy baseball, it’s one of the reasons I enjoy the activity. Rookies come out of nowhere to make you second-guess and adjust your evaluation methods for young players. Veterans leave trails of crumbs strewn along the field of play that in retrospect of course highlight the potential for a bounce-back season you never saw coming. But for all the new things you can learn that will (hopefully) make you a better player over the long haul of a dynasty league, sometimes paying attention to the most basic and fundamental of lessons can go a long way towards helping you avoid the kind of dumb mistakes that saddle you with bad contracts and lead to poor roster construction. One of those primary lessons: while speed may never slump, it definitely gets a whole lot less speedy when a player passes his prime. Sure, the occasional Juan Pierre will throw up a 60 steal season at age 32, and Rajai Davis looks like he’ll be good for 40 bags a year until he’s collecting social security checks. But overwhelmingly, players who rely on speed for the bulk of their value decline rapidly at the end of their twenties.

The 2013 seasons of Michael Bourn, Carl Crawford, and Brett Gardner provide three cautionary tales on post-prime speed, and they are worth a brief examination today.

Bourn had done nothing but steal bases at an elite clip for his entire career. Entering 2013 he’d been healthy, coming off back-to-back 700+ PA seasons. And while his 42 steals in 2012 were a decided step back from his peak seasons, he still ranked fourth in baseball in bags and appeared prime for another strong season in his new Cleveland digs. Crawford for his part entered this season as a nice under-the-radar bounce-back candidate. Coming off two straight injury-plagued, unproductive seasons, his previously elite fantasy skillset offered an intriguing “buy low” opportunity. And Gardner made for a nice upside flyer. He missed most of last season with an elbow injury (aka a non-leg injury), was coming off back-to-back elite seasons of 47 and 49 steals before his lost 2012, and would be a likely candidate to lead off for the always-potent Yankee lineup.

But a funny thing happened to all three in 2013: they stopped being elite base-stealers. One of my favorite quick and dirty metrics to evaluate speed for fantasy purposes is SB/PA. It’s a good way to get an eyeball expectation for how many steals you’re likely to get out of a player over the course of a full season. Your typical ~20 SB player will average somewhere around a steal every 25-30 plate appearances, slightly less than 1 a week. The elite base-stealers, the 40+ guys, will check in at half of that rate or better. The best of the best (non-Billy Hamilton division) will typically come in somewhere in the 11 to 14 per-plate-appearance range.

Through their respective age 28 seasons, which for Gardner was last year, Bourn in 2011, and Crawford in 2010, here’s how those three players ranked for this metric:

Michael Bourn: 2,663 PA’s through age 28, 234 SB’s, 11.38 SB/PA
Brett Gardner: 1,619 PA’s through age 28, 137 SB’s, 11.81 SB/PA
Carl Crawford: 5,395 PA’s through age 28, 409 SB’s, 13.19 SB/PA

Aside from the grandeur of Carl Crawford amassing almost 5,400 Major League at-bats before his 29th birthday, the takeaway here is that we are, in fact, dealing with three elite thieves, at least as judged by peak performance. Problem is they’ve all passed those peaks, and the picture on the down side of the hill is not a pretty one when gauging the future value of these players. Since the odometer hit 29 here’s how these three have performed on the bases:

Bourn: 1,207 PA’s, 64 SB’s, 18.86 SB/PA
Gardner: 595 PA’s, 23 SB’s, 25.87 SB/PA
Crawford: 1,085 PA’s, 36 SB’s, 30.14 SB/PA

In Bourn’s case, his age 29 season last year saw a decline of about 5 PA’s/SB to 16.73, and he’s posted a subsequent regression to a 22.91 rate so far this season. Meanwhile, it’s a small sample size for Gardner – this is his age 29 season – but a clear drop in production so far this year despite ample opportunity to run. And even if we chalk up some of Crawford’s decline to injury/general ineffectiveness of approach in Boston, his rate of 32.38 this season in LA represents a startling fall from grace.

Once a speed guy starts declining, the dollars to donuts likelihood is he keeps declining. In all but the deepest of keeper leagues I would look elsewhere for speed options going forward. If the opportunity presents itself to flip any of these guys to an unsuspecting manager off their name recognition and reputation this off-season I’d look to capitalize on the opportunity right quick. And for future drafts do yourself a favor and remind yourself to be wary of over-valuing past performance on bases for guys heading into their thirties. Speed is a fickle and fleeting commodity, and it’s usually important to cut bait before it’s too late.