Considering that Mendez speaks in clipped sentences, it seems likeliest that he meant Pike at timepoint X was the age Kirk is now. Remember how the exchange goes? Mendez asks if the two have met; Kirk says they met at Pike's promotion; and then
Mendez launches into this line where he describes the man Pike was.
It would make sense for Mendez to emphasize how alike Pike and Kirk are, despite their considerable age gap - both in terms of the plot of the episode (Mendez wants to tell Kirk what a loss Pike's injury is to him and Starfleet) and of the drama (the audience draws the obvious connection between the two skipper characters, separated by decades but subject to similar whims of fortune - they already care for Kirk, but now they have to start caring for Pike, too, and that's easy because Pike is
I can't for a moment think that the writer would literally have meant to say that Pike is the same age as Kirk. He was
the same age once, though, and what we hear is a valid expression of that.