Anakin couldn't have been 18-20, because Luke was roughtly that age in ESB according to Yoda, and Yoda called him "too old to begin the training". That means that even in the OT, Jedi were envisoned to start their traning at a fearly young age. Probably no older than 15 at the very least.

Lucas's first story ideas for The Phantom Menace included Obi-Wan finding Anakin on Tatooine. Qui-Gon was a Jedi friend that Obi-Wan met later at Coruscant. Lucas apparantly disregarded this plot idea because he didn't want the young Anakin to se Obi-Wan as a father figure, since they are more like brothers. Qui-Gon then stepped in to become Anakin's "father" for a brief period.

Even though I like the idea of Qui-Gon finding Anakin, I must say that I would prefer if Qui-Gon and Obi-Wan were friends in TPM rather than Master/Padawan. It could be sort of like Obi-Wan and Anakin's friendship in ROTS.

If we are in the field of hypothesis then maybe Yoda could've said "too old" since that mistake was taking a 18-20 year old Anakin and doesn't want the same thing to happen again with Luke. But while Jedi could have been like a school from what we hear in th OT, in the PT they are portrayed as *stealing* (or taking or asking) babies from their families in a much earlier age in a more religious manner. Obi-Wan is the quintessential father figure to Luke, it could have been like they are in AOTC or ROTS. As you say it would be interesting, but that level of editing I guess it would be PRETTY hard to accomplish....is almost like making a brand new movie.