Kenobi was a headstrong adolescent, who longed to become a Jedi. As is custom, when a hopeful achieves the age of 13, he must become a Padawan. Otherwise, he is too old to proceed in his training. Washouts are instead sent to the Agricultural Corps where their Force talents are directed to tending sick crops rather than protecting peace and justice.
A small snippet from Obi's bio at the database. I found some more info on the official forums too.
There hasn't really been any more written about this. It's called the Agricultural Corps, and the only references to it that I know of are in the Jedi Apprentice series of kids' novels.
http://www1.theforce.net/cuswe/search.asp?search=agricultural+corps&EandD=EyDn&no=10)
Here's what I got from the official SW board.
Agricultural Corps...
You can find out more about the Jedi Agricultural Corps (AgriCorps) on wizards.com.
According to the Dark Side Sourcebook, High Prophet Jedgar from the Prophets of the Dark Side young adult novel was a Jedi hopeful chosen for the AgriCorps when he was 13, but he fled and eventually fell to the dark side.
One last little bit of info I got.