Right after Disney raised the age to 14 we tested it with our then 13-year-old. We hung back as she walked up by herself and went in at all 4 parks, no questions asked.
As others have mentioned, the reason why Disney changed it to 14 was because they had parents out in DL using it as daycare. I think if your child is middle school aged and doesn't cause issues they would never be questioned. If they go in and act unruly, Disney wants the right to kick them out.
Since we tested it, I haven't thought of it, but last October, my DD 12, went through bag check. I just walked through to the gate at HS. We were no where near each other and no one stopped her with her magic band, that I think still says she is 10! As a family we rarely go through the same gate, everyone wants to see who can pick the best line!
My younger DD won't be 14 until she is in High School, I personally feel the age Disney set is a little too high, but I don't think they enforce it unless there is a reason to.