Judges aren't necessarily brilliant, but they do spend their entire careers reading, listening to, and dissecting arguments. A large part of this requires learning new information at least well enough to make sense of arguments on both sides of the issue. So you do end up probably self-selecting for older folks able to do this better than the mean for their age, and likely for the population at large.

Let's just say with a full jury you're almost guaranteed to get someone on the other side of the spectrum, regardless of age.