ESPN reports that the Nationals and Ryan Zimmerman have agreed to a 6-year, $100-million contract extension, with a full no-trade clause. The deal is for the years 2014-19, when he’ll be age 29-34.

That’s right: Zimmerman was already signed through next year for a total of $26 million.

Now, there’s plenty to like about Ryan Zimmerman: good hitter, good fielder, face of the franchise ever since he was the #4 pick of the loaded 2005 draft and was in the majors for good that September. He had 110 RBI as a 21-year-old rookie; only 21 others have had a 100-RBI year that young, and most of them are in the HOF or headed that way. He’s hit better at home, which is no mean feat in the parks he’s played in. He’s been a ROY runner-up and an All-Star, won a Gold Glove and two Silver Sluggers. There’s never been a Washington Nationals season without him.

Still, there are two obvious questions:

• Will he be worth $100 million over those 6 years?
• And why sign a new long-term deal two full years before his old deal runs out?

I’m not here to answer those questions, only to start a discussion. Let me raise the easier question first: Why now?

As you’ll see in a moment, I think Zimmerman’s performance during the 6-year deal is unlikely to match his play thus far. But there’s also the question of his durability. Zimmerman has  missed big chunks of time in 2 of the last 4 seasons, including last year, and his performance in both years was not up to his usual standard. Why commit to a full 8 years down the road for a player who missed 2 months last year?

Let’s look at Zimmerman’s value over the past 3 seasons. That’s the timeframe most advantageous to Zimmerman, taking in his 2 best years by far (2009-10). Here’s where he stands among all position players in that time:

• bWAR, 20th, 12.8.
• OPS+, 22nd, 132 (min. 1,000 PAs).
• Games, 98th, 400.

Now here’s something that boosts Zimmerman’s immediate value: Good hitting third basemen are fairly scarce these days. In bWAR and OPS+, Zimmerman is 2nd only to Evan Longoria over the last 3 years.

Well, OK — but does today’s shortage predict what we’ll see 2 to 7 years from now? I think not.

Zimmerman’s two best WAR seasons were 5.3 and 5.2, at ages 24-25. Those were real good years. But this deal isn’t about then, or even now; it’s about ages 29-34. And as the opening chart shows, the number of star seasons by third basemen historically drops sharply through that range. Here’s a similar chart of 3Bs’ high-OPS+ seasons by age:

Now I’ll try to anticipate how Zimmerman might hit during the new contract by looking at groups of hitters similar to him through age 26. Each of these tests covers 1901-2003, stopping there so that all players included will have reached age 34 by now.

First, a conservative approach taking in his full career to date. Zimmerman so far has a 120 career OPS+ in 3,669 PAs. I looked at all hitters through age 26 with an OPS+ from 115-125 and at least 2,500 PAs (from 1901-2003). There were 72 such players; their average OPS+ was (surprise) 120.

• For age 29-34 combined, this group had an average 109 OPS+ (unweighted), 11 points below the original baseline.

The second approach focuses on Zimmerman’s last 3 years, age 24-26, with a combined 132 OPS+ in 1,737 PAs. So I looked at hitters age 24-26 with OPS+ of 127-137 and at least 1,500 PAs. There were 73 such players, with an average OPS+ of 132.

• From age 29-34 combined, that group averaged a 121 OPS+, 11 points below the original baseline.

Thirdly, I looked at all 3Bs with at least a 110 OPS+ and 2,000 PAs by age 26. This is the cream of the young 3Bs, a group of 29 with an average OPS+ of 127 — better than Zimmerman’s 120.

• From age 29-34, they averaged a 117 OPS+, 10 points below the original baseline.

Finally, a WAR-based approach with a defensive piece. Zimmerman so far has 19.8 WAR and a dWAR of 0.2. His defensive reputation is better than that, so I looked at all 3Bs through age 26 with at least 15 WAR and positive dWAR. This group of 23 has 8 HOFers and a few more on the way; their average WAR was 22.7.

• From age 29-34 combined, they averaged 17.5 WAR, or 2.9 WAR per year. Eight of the 23 had less than 10 WAR in those years.

No matter the method, it seems that most players of Zimmerman’s caliber — as hitters and all-around — have been lesser players from age 29-34 than they were by age 26.