I don't encourage my Scouts to set thresholds for leadership positions. I encourage my Scouts to think about what the leadership position entails and vote for the Scout who can get it done.
Last year we had a Scout who had been with our Troop for 6 years and just passed the swimmers test for First Class. He just had mental block about it. He has mounds of camping experience, great Scout skills and held several other PORs in the past. He was elected SPL and did a good job. If he had failed the swimmers test again should he have been denied the chance to run for SPL, I don't think so.
Our troop currently has two 12 year old Star Scouts. Are they ready to be SPL? I can't tell you for sure, and it is not my decision. The Scouts will pick who they think is best. They may make a horrible mistake but you know that is a lesson it itself isn't it.
My point is we don't need to put limitations on Positions of Responsibility we need to teach Scouts to think about the candidates and if they can get the job done.
Just my 2 cents.