Using the methodology of comparing players in adjacent seasons (good word, David, and it means comparing Tim Raines at age 26 and 27 for example), and seeing how his stats change, and doing this for all players in MLB from 1919 to 1999, here is how the stats breakdown for ages 21 to 37: Here are the 1979-1999 age factors Age PA1,,,, $BB, $K,, $HR, $H,, $XBH $T,, $SB, $LW,, 21 14,627, 0.62 1.24 0.69 1.00 0.87 1.44 0.83 0.900 22 41,705, 0.74 1.20 0.74 0.96 0.92 1.11 0.95 0.914 23 81,533, 0.74 1.12 0.82 0.95 0.94 0.95 0.92 0.939 24 139,061 0.79 1.04 0.89 0.94 0.95 0.91 1.00 0.967 25 188,156 0.82 1.01 0.89 0.93 0.99 0.83 0.93 0.976 26 226,485 0.84 1.01 0.96 0.93 1.00 0.75 0.90 0.996 27 231,667 0.88 1.00 1.00 0.92 0.99 0.69 0.87 0.999 28 219,253 0.90 1.00 0.97 0.89 1.00 0.60 0.82 0.978 29 201,555 0.92 1.02 0.97 0.88 0.99 0.54 0.79 0.970 30 185,070 0.94 1.02 0.93 0.86 0.97 0.54 0.71 0.949 31 156,383 0.96 1.04 0.95 0.85 0.96 0.50 0.64 0.940 32 130,953 0.96 1.04 0.90 0.84 0.95 0.44 0.62 0.922 33 112,054 0.97 1.06 0.86 0.82 0.91 0.41 0.57 0.896 34 85,403, 0.97 1.09 0.85 0.80 0.89 0.38 0.50 0.873 35 64,545, 0.96 1.13 0.82 0.77 0.89 0.32 0.44 0.840 36 45,277, 0.97 1.14 0.80 0.76 0.90 0.35 0.42 0.828 37 30,083, 1.00 1.13 0.72 0.75 0.89 0.34 0.35 0.808 38 20,304, 0.93 1.16 0.63 0.73 0.86 0.20 0.35 0.754 39 13,183, 0.89 1.26 0.52 0.72 0.79 0.26 0.30 0.707 One little note is the "T". Because of the sample size, I'm sure the triples are probably off. So, rather than base it against the peak level being 1.00, I forced in a .69 level at age 27, to match the historical average. As you can see, the overall peak was also at age 26.5. I will republish the 1919-1999 factors, cause I found a little bug with my $H calc: Age PA1,,,, $BB, $K,, $HR, $H,, $XBH $T,, $SB, $LW,, 21 76,865, 0.66 1.32 0.68 1.00 0.93 1.00 0.87 0.890 22 175,651 0.70 1.24 0.78 0.98 0.94 0.94 0.99 0.914 23 310,970 0.76 1.14 0.88 0.99 0.97 0.93 0.92 0.963 24 453,555 0.80 1.08 0.91 0.98 0.98 0.85 1.00 0.980 25 583,518 0.83 1.05 0.95 0.97 1.00 0.79 0.96 0.991 26 667,077 0.86 1.03 0.98 0.96 1.00 0.76 0.92 1.000 27 698,654 0.88 1.01 1.00 0.95 1.00 0.69 0.88 0.999 28 672,429 0.90 1.00 0.99 0.93 1.00 0.67 0.84 0.986 29 624,283 0.92 1.00 0.98 0.91 0.99 0.62 0.80 0.975 30 567,730 0.94 1.01 0.96 0.89 0.98 0.58 0.75 0.953 31 481,548 0.96 1.02 0.95 0.88 0.97 0.54 0.66 0.945 32 392,880 0.97 1.03 0.92 0.86 0.94 0.51 0.63 0.924 33 321,931 0.97 1.05 0.87 0.84 0.93 0.49 0.58 0.896 34 240,763 0.98 1.06 0.86 0.82 0.89 0.45 0.52 0.874 35 170,445 0.97 1.11 0.80 0.80 0.88 0.42 0.47 0.842 36 114,739 0.98 1.13 0.76 0.77 0.88 0.39 0.45 0.815 37 73,584, 1.00 1.14 0.71 0.75 0.87 0.35 0.38 0.789 Other than the triples as I noted, the historical averages much up very well with the recent period. While today's ballplayers may be better, and playing longer, the "curve" of their aging is the same. There is no age bias with today's regimen of training and medication. It affects all age groups the same. I haven't done any adjustments, and so this is definitely a first-look, and not the last look. Lotsa stuff here might change as I refine all this. Anyway, here is what the columns mean: Age: the age of the player as of July 1, rounded to the nearest day. Age 26 is all those players aged 25.5 to 26.5. PA1: the plate appearance of the smallest of the adjacent seasons. If Raines has 400 PA in 1994 and 300 in 1995, the PA would be 300. These columns might change, as I hear more from you guys as to whether you like these definitions. I borrowed the concept from DIPS. $BB: BB/AB. What I did here is compare BB/AB for age 22 to the BB/AB for age 21. This ratio 1.07. I did this for all pairs of years, and then chained them. The result on the table is % of peak level. In this case, the peak level for walks was age 37. $K: K/(AB-K) $HR: HR / (AB-K - HR) $H: (H-HR)/(AB-K-H) $XBH: (2b+3b)/(1b) $T: 3b/2b $SB: SB/(.8*1B+.6*BB) $LW: plus LW / hitting outs (i.e., plus LW = .47 for a 1B, 1.41 for a HR) Now, you could use these numbers to do your projection stats, etc. I'm sure all you projectors out there use something similar. Just wanted to add another table to the mix. Anyway, hitters always improve their walk ratio, they strikeout the least at age 29, get their best HR ratio at age 27, their balls in play success goes down almost instantly, their line drive power stays pretty flat for a long period of time, their speed as measured by triples goes down instantly, their speed as measured by SB peaks at 24 and goes down almost at the same rate as the triples. The overall peak age is 26.5 As I said, I just did this right now, and I'm still looking to refine it.