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Why Merrill Kelly Will Defy the Aging Curve

Young in spirit, sharp of mind, & strong in body.

Merrill Kelly.
Merrill Kelly.
Photo by Carmen Mandato/Getty Images


Last season, Merrill Kelly was a clear # 2 in the rotation, with the second best FIP and the best ERA. His percentage of quality starts was tied for the 14 th/15th best in the Majors. He and Gallen were the best in five measures of pitching consistency.

When I looked at the ZiPS projection for the Diamondbacks’ starting pitchers (including the newly acquired Eduardo Rodriguez), I compared projected ERA for next season to actual ERA in 2023. The biggest worsening was Merrill Kelly (3.97 projected ERA vs 3.29 actual ERA). What made that especially surprising is that a big characteristic of Merrill Kelly is consistency. For example, last season his ERA was consistently better than the 3.97 ERA that ZiPS projected:

  • 3.22 ERA, first half regular season
  • 3.38 ERA, second half regular season
  • 2.25 ERA, postseason

Merrill Kelly Strengths

He makes needed adjustments every season.

His best pitch was different in each season. That is a strong indication he is making adjustments to stay ahead of batters. One reason this works for him is that he uses 6 different pitches.

  • 2019 Curve
  • 2020 Sinker
  • 2021 Curve and cutter
  • 2022 Cutter and changeup
  • 2023 Changeup

Making adjustments is a reason he will likely defy the aging curve.

His pitch velocity is stable.

The following table shows that his pitch velocity was stable from 2019 to 2023.

Data from Baseball Savant.

“As long as a pitcher is able to maintain a certain velocity, like Halladay has, the player can generally pitch with the same results year after year. It’s only when the pitcher begins to lose velocity that he sees his stats degrade at a higher rate. Sure, velocity isn’t everything with a pitcher — but it’s important. Pitchers who maintain their velocity don’t really “age” — they stay the same.” — Jeff Zimmerman, 2012

Similar to what Jeff Zimmerman said over ten years ago, as long as Merrill Kelly maintains his velocities, he is likely to have the same results, thereby defying the aging curve.

His ERA+ increased.

His ERA+ increased from 119 in 2022 to 132 in 2023. An increasing ERA+ shows he is defying the aging curve.

His whiff percent increased for several pitch types.

Last season, his whiff percent was higher for his 4-seam fastballs, changeups, and cutters.

  • 28.0% to 32.0%, Changeups
  • 25.4% to 27.6%, 4-Seam Fastballs
  • 20.8% to 24.4%, Cutters.

Increased whiff percentes are a reason he is defying the aging curve.

His batted ball profiles are not degrading.

The following list shows that his batted ball profiles in 2023 were as good as they were two seasons ago.

  • 2021. 36.0% Hard Hits, 34.4% Fly Balls, 21.6% Line Drives, .126 HR/FB
  • 2022. 28.5% Hard Hits, 37.9% Fly Balls, 19.3% Line Drives, .099 HR/FB
  • 2023. 35.9% Hard Hits, 34.1% Fly Balls, 20.7% Line Drives, .128 HR/BF

Batted ball profiles show he is defying the aging curve.

His quality starts rank high.

Last season, his 58.8% quality starts ranked in a tie for 14th/15th best in the Majors (data from Team Rankings and MLB. com, stats combined regular season and postseason starts). The percentage was better than his 54.6% in 2022. The high ranking in quality starts shows he is defying the aging curve.

Let’s look at the average number of quality starts for older pitchers. For last season’s top 43 starters by WAR, the following table shows the average number of quality starts for pitchers aged 33 and above (Kelly was 34 last season).

Data from Team Rankings.

Although ten pitchers is a small sample size, the table suggests a 10% decline in number of quality starts at age 35 and an additional 20% decline at age 36. Although Merrill Kelly will be age 35 this season, his 20 quality starts suggests that he is pitching like a younger pitcher (and that his decline will be delayed by at least a year).

Pitchers Who Defied the Aging Curve

Much has been written about three pitchers who defied the aging curve - Alex Cobb, Jacob deGrom, and Justin Verlander. Each of them was exceptional. For example Jacob deGrom’s average fastball velocity at age 35 was 98.7 MPH.

Alex Cobb and Justing Verlander had something in common. Each of them started their pitching careers with exceptional leg strength, and at a later point in their career benefited from focusing on strengthening their upper body.

The following quotes led me to my conclusion.

[After Tommy John surgery] “Cobb started building strength in his upper body by lifting more weights, learning that the long-held assumption that more muscle mass can lead to higher injury risk doesn’t hold much scientific merit. He did more cardio on the stationary bike instead of running — standard practice for pitchers — because it’s less strenuous. He listened to wellness podcasts and embraced cold-hot therapeutic treatments for recovery.” — Danny Emerman

After he experiened a velocity drop in [2014/2015?], Justin Verlnader looked for help from a personal trainer. “He’ll probably kill me for telling you this, but I was surprised — I had junior high kids that were stronger than him in the gym, in the upper body. His legs were always strong. But it was ridiculous how fast he got strong, once he was doing the right stuff. Just because his work ethic is so good.” — Peter Park, personal trainer

And Jacob deGrom talked about the importance of maintaining upper body strength during the season. He builds strength in the offseason.

“JACOB DEGROM: During the season, it’s more just maintaining. As a pitcher, it’s important for your shoulders, rotator cuffs, and back. That’s my main focus. I keep a good shoulder routine and [scapula muscle] routine. But Noah’s more into working out than I am. Free weights is mainly what I use.” — Jacob DeGrom

The following quote explains why pitchers have better chances than batters to defy the aging curve. My view is Merrill Kelly is one of those pitchers who defy the aging curve.

“I think when it comes to aging in the game, pitchers have the advantage over hitters. It’s not as much of a reaction sport. Reaction time is probably the first thing that goes as you age. We don’t rely as heavily on that. The physical ability — I don’t know why that would go away if I’m continuing to see my strength and conditioning stay the same, my health stay the same. We’ve seen guys like Charlie Morton, Rich Hill, (Max) Scherzer, (Justin) Verlander — a lot of guys have paved the way to show that your career isn’t ending at 35 if you do the right things to maintain your health.” — Alex Cobb


Despite the ZiPS projection, despite that he is 35 years old, we considered six ways that Merrill Kelly defies the aging curve. The most compelling are he makes adjustments every season, his velocity is slightly better than 2019, and he ranks high in quality starts.

Next season, he will very likely continue to defy the aging curve. Although he may not reach his posteason ERA of 2.25, next season a reasonable prediction would be a 3.29 ERA, the same as last season.