It's an explicit and unqualified statement by one of the characters - all attempts to make it fit with the rest of what's established in the episode require rationalizations and debatable interpretations, but the line itself stands as an unambiguous statement.
But it's only a contradiction to the rest of the story if one reads it in a certain way. And reading it the other way is not merely a "legitimate way out", it is in fact a likely, rather commonplace and fully natural way for the writer of that line to have expressed his intention that Pike be older than Kirk.
IMHO, it's a gross reading error rather than a writing error if one thinks Mendez there is saying that Pike is the same age as Kirk. The verb in that sentence is not in the present tense, but is missing, so we might not immediately realize that it is supposed to be in the past tense. Pike was the same age as Kirk. Pike was a strapping hunk in the prime of his life. He is neither of those any more, which is the very point of Mendez' line.
Timo Saloniemi