^
^^ 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.
[...]
EDIT TO ADD:
I went to the transcripts and found this:
[scene here]
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.
I might also be inclined to suspect late script revisions or line cuts as a culprit -- a few obvious goofs made it on-screen, probably due to looming production deadlines -- but
looking at this, I see nothing which indicates that any changes or cuts were made to the script in the exchange between Mendez and Kirk there.
Really, though, this isn't a detail I worry about much.