Why is Kasey Keller always "old" or "aging"

I saw this post at The Offside previewing Seattle Sounders FC. And it included this comment:

As it stands, the offense looks like it won’t have much trouble scoring, but the defensive remains a question mark in front of proven — but aging — keeper Kasey Keller. Nevertheless, MLS is coming to Seattle and they sure look like they are ready for it to be there.(emphasis added)

. Kasey Keller is 39 years old. Is that old for a professional footballer, yeah, maybe, but not so old for a goalkeeper. Keller is not even the oldest first team keeper in MLS. Pat Onstad, the first team keeper for Houston just turned 41 and no one is saying that his career is over.

Here are the likely first team goalkeepers in the MLS (not saying this is a lock, but likely)

Jon Busch (Chicago) 32
Dan Kennedy (Chivas) 26
Wil Hesmer (Columbus) 28
Preston Burpo (Colorado) 36 (Matt Pickens is 26)
Dario Sala (FC Dallas) 34
Louis Crayton (DC United) 32
Pat Onstad (Houston) 41
Kevin Hartman (Kansas City) 34
Donovan Ricketts (Los Angeles) 31
Matt Reis (New England) 34
Danny Cepero (New York)* 23 (Jon Conway is 31)
Nick Rimando (Salt Lake) 29
Joe Cannon (San Jose) 34
Greg Sutton (Toronto) 31
Kasey Keller (Seattle) 39
*Cepero will start probably the first 10 games since Conway has a ten game suspension for performance enhancing drug use. After that it will be a matter of Cepero's performance to see whether he stays where he is at.

As you can see, the above list (not counting Pickens who could start). there are 15 teams here, here are the stats

Youngest-- Danny Cepero (23)
Oldest-- Pat Onstad (41)
Average Age--32.6.
Median Age--34 (Cepero's age skews things).

If you put Conway in you get these stats
Youngest: Dan Kennedy (26)
Oldest: Pat Onstad (41)
Average age: 33.5
Median Age--34.

As you can see, Goalkeepers tend to be a bit older. The position requires as much, if not more, experience and knowledge than pure athletic ability. Not to say that these men are not good athletes, they clearly are. I am just saying that it doesn't require 90 minutes of running. But it does require 90 minutes of concentration since, as a keeper you might go 89 minutes without doing a thing, but in that 90th minute you have to make the one save to keep your team from losing or drawing at the last minute.