There's a Baseball Prospectus article that I've been referencing since 2006 because it gives the average July 1, 2006 age at each minor league level. How about something a little more recent?
The data below is from a April 17, 2009 snapshot of the minors and majors. Standard deviations are listed in parenthesis.
Average age by level
MLB: 29.5 (4.2)
Triple-A: 27.4 (3.2)
Double-A: 24.9 (2.0)
High-A: 23.3 (1.6)
Low-A: 22.2 (1.6)
The data from the BP article that I referenced above is extremely similar to our data. Clay Davenport didn't provide a MLB age or standard deviations, so we can't match everything up -- though he did include short-season (21.9) and rookie ball (21.3) average ages. Again, BP's ages are from July 1, 2006 opposed to our April 17, 2009. Here's a quick comparison:
BP/PP
Triple-A: 27.4/27.4
Double-A: 25.0/24.9
High-A: 23.5/23.3
Low-A: 22.3/22.2
A couple comments
Some people may say that Matt LaPorta is "a little old for a prospect," but he still was nearly a full deviation below the average Triple-A age this season.
Rick Porcello opened the 2009 season at 2.2 deviations younger than the average big leaguer.
Player examples
Here are some April 2009 average age players from each level:
MLB: Jason Bartlett and Khalil Greene
Triple-A: Jeremy Accardo and Sam Fuld
Double-A: Travish Chick and Matt Camp
High-A: Brandon Waring and Brian Rike
Low-A: Dexter Carter and James Darnell
The 10 oldest big leaguers
Again, as of April of 2009:
1. Jamie Moyer, 46.4
2. Randy Johnson, 45.6
3. Tim Wakefield, 42.7
4. Omar Vizquel, 42.0
5. Matt Stairs, 41.1
6. Brian Shouse, 40.6
7. Russ Springer, 40.4
8. Gary Sheffield, 40.4
9. Brad Ausmus, 40.0
10. Troy Percival, 39.7
Thanks to Sean Halloran for helping us gather this data. Feel free to email comments or questions about this data to adamf@projectprospect.com. I'm also happy to share my age spreadsheet.