How can you determine an age for that? There are some who as 8 through 10 might not have the attention span to be in the infield safely. Then all of a sudden they get a little maturity and are awesome in the infield. The opposite is also true, I've seen kids who have played infield for years, mostly because of a good arm at a young age, who then don't have the reaction or instincts for the infield as they get older, but make awesome outfielders.
I hope and believe all the members in this forum places equal value on both. I have seen coaches utilize one player by switching them between the infield and outfield for certain games. My dd plays infield 95% of the time, but has great speed and arm for centerfield. My thought is that there is a difference between the response time needed in a hard hit line drive to third and a fly ball to centerfield. Hideki Matsui advises the fielders to keep their eye on the bat because once that ball is hit, every player needs to be moving.
Here is the semi-scientific answer and I am not saying it's the whole absolute thruth as there are many variables.
Softball is a late specialization sport (i.e. you peak at later age unlike gymnastics for example). For that kind of sport, it is recommended to start specializing between the ages of 13-15. It's correct to have a primary position (that you are super familiar with and train at most of the time) and a secondary position (that you are familiar enough to do well at and train at on a regular basis).
Those two positions can be one in IF and one in OF. OF starts to become "important" (and not seen as a punishment anymore) around 14U or 16U. Selection of a position will depend on many factors - athletic attributes of the athletes, team needs, athlete's personal preference, etc.
So clear answer but while it is highly recommended that you play multiple position until the age of 12-13, you should start thinking about specializing.
Guess what? they still learn new position at the college level and on international teams.
Why? Because many great athletes all play the same positions (SS, CF are very common) once they get to that level and coaches have to make choices as to who will be a SS and who will get to be a 3B or 2B or RF.
Thank you Mark. That is the feedback I was looking for. I oversee a Rep program in my community and was having a problem with concerned parents. You have reaffirmed what I believed to be true. Just wanted a second opinion before I wade into the dispute.