Zabat’s scale is pretty good, with the caveat that if you hit the upper limits of it, people start to qualify performance with “but he was 22 in High-A, so we’ll see what happens when he’s tested at a more age-appropriate level.” The last age in each of those levels is where age starts to be discussed as a bit of a negative, with the exception of triple-A, where a lot of people stick around until age 24-25 due to lack of opportunity at the big league level, which shouldn’t reflect on their quality as a possible major league hitter quite as much.

The scale that BA makes a lot of their assumptions on is that “standard prospect age” operates something like this:

Low-A: 19-20
High-A: 20-21
Double-A: 21-22
Triple-A: 22-23

That’s the “right on track for a prospect” age relative to league assumptions. Anyone younger gets their age counted as a pretty big positive, and anyone older gets it counted as a bit of a negative as far as prospect status is concerned, with the caveat that individual players can figure things out at any time and at any level and turn into major leaguers.

The other thing is that age doesn’t matter anywhere near as much for pitchers as it does for hitters. This is partially because pitchers are so unpredictable, and partially because their age curve looks very, very different. It’s not at all uncommon for pitchers to peak at 23-25 rather than 27-30 for hitters.

For that reason, A 23-year-old who all of a sudden starts blowing away Midwest League hitters with an upper-90’s fastball, a sharp-breaking curveball, and a devastating changeup needs to be tested at higher levels, but you don’t discount his performance as merely a function of being older and more advanced. Major league stuff is major league stuff, no matter how old the arm that throws it is. All age tells you is the chance that this kind of player will turn in a hall-of-fame career. It doesn’t effect whether or not they should be considered potentially useful major leaguers. When these guys emerge as true major league talents they tend to fly, though. This is what happened to Mark Lowe during his meteoric 2006 season that saw him begin in High A as a 23-year-old and end with a 17 inning scoreless streak in the majors. Sticking with Seattle, it was pretty apparent that George Sherrill had a chance to be a true lockdown reliever with his stuff even though he was 28 and in Triple-A.

On the flipside, an 18-year-old succeeding in high-A with a mid-80’s fastball and a solid-average secondary stuff is interesting, but his age shouldn’t give him much of a boost. As risky as pitchers are, it’s foolish to dream on any of them or value them very highly until they have present major league stuff to go with their good pitchability. Yes, many projectable types see a huge uptick in their stuff, but it’s really not a good bet on any individual player. Deolis Guerra sparked huge debates a couple years ago because he had a sparkling ERA in the 2’s in full-season ball at a young age, but nothing in his repertoire of pitches suggested he could get major league hitters out until his stuff improved dramatically. It didn’t, and now he’s a fringe prospect at best.

They don’t have it yet for 2009, but average ages for pitching and hitting separately, for every league, for 2008, are here:

http://www.baseball-reference.com/minors/league.cgi?year=2008

I’ll add, the short season leagues are where guys tend to get sent after being drafted in June. A college junior is typically 21 when drafted. The top college players get sent to the top short season leagues. They maybe start the next season in A+, at age 22, if they are advanced enough, or could be in A- if they are less advanced.

If a guy is very recently signed or drafted, you have to be careful about saying they are too old for a level. Maybe you don’t get too excited about a guy putting up numbers in low A at 23, but it doesn’t mean he’s too old to be a prospect either, especially if he was just drafted. For a guy who was recently drafted, where he was drafted is often a good place to start as far as an indication of how highly regarded he is.

So I’m saying, you should take experience into account as well. Age is certainly a useful indicator, I’ve just seen it taken to extremes at times, to the point I’ve seen people say silly things like “we shouldn’t draft college players, they are too old”.

- College players operate on a different development curve than prep players. This would seem intuitive as the college game is very different from the pro game, but tends to get overlooked sometimes. I don’t start REALLY hating on a college player for not measuring up until he’s into his second full pro season.

- Sometimes, players need a lot of time to figure things out . . .but once they do, watch out. Sometimes guys take a long time to get out of A ball and then they take half a season from there to get to the majors. Sometimes guys spend a few years in AA learning how to play. Patience is a virtue . . .this is a big reason why scouting reports are so important.

- ARL is nearly useless for pitchers, in my opinion. Somebody else mentioned this above, but if a guy has the raw stuff to get by major leaguers, where exactly he happens to be honing his craft in preparation for it isn’t always a big deal. Pitchers need their time against good competition to get ready for the majors, but if a guy is 22-23 and still putting things together in A ball, it’s really not a big deal. A 95 MPH fastball is a dangerous weapon no matter who it’s being thrown to.