wait i'm not done.
this man is an ALIEN. he meets Batman AS AN ALIEN BEING. imagine knowing Superman for like, a year or so, enough that this friendship is solidifying, and his age/birthday comes up (maybe because he insisted on getting something small for Bruce's more publicly known birthday!) and this adult alien who is NOT BEYOND pulling petty shit when it's harmless and funny, says with a straight face and no explanation, that he's six years old. what is bruce supposed to do with that? maybe kryptonians age differently! clark could draw it out for a while, letting Bruce think he's younger than Robin! and Bruce wouldn't have a good reason to disbelieve it unless he like, went digging for Clark's high school records or something (which he MIGHT do but then maybe he's trying to respect a little of the privacy of the one friend he managed to make, pre-intense paranoia days).
and then it was just a leap day joke