Why did Obi-Wan wait so long?

If the Jedi were supposed to be trained since infancy, like they are in the prequels, and Obi-Wan can conclude that one of the many reasons for Anakin's turn to the Dark Side was his (by comparison) old age, then WHY did he wait till Luke was almost TWENTY to begin his training?? Maybe if Jedi training began later, than we could justify things, but that's a pretty big margin of error. Well, whatever, looks like Lucas forgot about what he wrote again...

GK edit: Please read the lovely Index before creating a new thread. Here is a very similar topic.