This seems less about how old you look and more about how young your husband looks. If he is old enough to have his own child but is being asked if he’s the older brother of a newborn, it sounds like he probably looks insanely young for his age, and judging by the fact that you said he is “21 months older” I’m assuming you’re both relatively young parents. Some men can look like young teenagers until their mid to late 20s, especially if they don’t have thick facial hair and shave what they do have.

Tired, I guess.