More importantly, Mendez was in the habit of using clipped phrases. He didn't say "Chris Pike is about your age, Jim". He responded to Kirk's statement of meeting Pike when Pike was promoted to Fleet Captain. And he responded with "About your age. Big, handsome man, vital, active."
I find it extremely difficult to believe that the writer of the episode would not have meant that Pike was about Kirk's current age back when he was promoted to Fleet Captain. That's the context of the phrase. Obviously, Pike isn't a vital and active man when the phrase is uttered. So there's no reason to think he would be Kirk's age when the phrase is uttered, either.
Pike is the older Kirk, a frightening reminder that this could be our Kirk's future as well - an example of cruel fate crushing the hopes of even the most heroic characters.
And Jeff Hunter looks the part, even if his shirt cuffs feature less brass than Kirk's. There's plenty of grey there, and much of it (all of it?) is probably makeup added to give the youthful Hunter an aura of experience. Whether the "live puppet" cast as Pike for the purposes of the two-parter was supposed to be of a different, less experienced age is doubtful. Sure, he's blonde rather than salt-and-pepper. But he's also scarred beyond recognition; for all we know, the doctors have given him young man's hair as inadequte compensation for his dreadful fate.
The basic issue remains: it's quite implausible that the writer would have attempted to indicate that Pike and Kirk are of the same birthyear. That's not something the episode would call for - Pike is supposed to be the precedent, the past, the Kirk-before-Kirk.
Now, we cannot nail down Pike's age by "The Menagerie" dialogue, because we have no proof of anything beyond him being alive and in Starfleet 13 years before the episode. At that point, he could have held any rank or rating, as long as he served alongside Spock in some capacity, and later became his CO and captain. Certainly there's no reason to start thinking that the character would be younger than Jeff Hunter in "The Cage", and little reason to insist even that he would have been as young.
Timo Saloniemi