"The Talosian flashbacks we are going to be seeing of him later in this episode took place back when he was 'about your age'."
That's what Mendez was trying to say. Of course, Kirk, having met Pike, knows exacly how old Pike is. The line was meant for us, not for Kirk.
No, I get that. I have always considered Pike during "The Cage" to be about Kirk's age. It was just a confusing way to introduce Pikes age IMO.