^
^^ Yeah -- I thought of this too, but when exactly would *then* be. At the time Mendez said that line, they did not even yet mention the Enterprise or the fact that Pike was once its Captain. They were not talking about a particular time in the past at that point in the conversation -- they were just talking about Pike.
That would be like me saying "President Bush -- he's about my age" when what I really mean to say is that President Bush was ONCE my age...well, yeah, if he is older than me then of course he was once my age.
EDIT TO ADD:
I went to the transcripts and found this:
MENDEZ: You ever met Chris Pike?
KIRK: When he was promoted to Fleet Captain.
MENDEZ: About your age. Big, handsome man, vital, active.
KIRK: I took over the Enterprise from him. Spock served with him for several years.
SPOCK: Eleven years, four months, five days.
MCCOY: What's his problem, Commodore?
MENDEZ: Inspection tour of a cadet vessel. Old Class J starship. One of the baffle plates ruptured.
MCCOY: The delta rays?
MENDEZ: He went in bringing out all those kids that were still alive. Just wanted you gentlemen to be prepared.
I suppose it is possible that Mendez meant that Pike was Kirk's age when Pike was promoted to fleet captain, but that doesn't really work either, since he lalready ooked older than Kirk during The Cage and was not yet Fleet Captain then.
I'll still go with Mendez (or Throne) mis-speaking...it happens.