They might have been too young and might have been sensitive to being caught. Or maybe they really were 21 and are used to people thinking they're too young. Either way, they're probably used to being looked at closely, with a skeptical eye, when buying beer.

I think my face is pretty easy to read, but my wife seems to have a lot of trouble with reading it. And I had trouble reading her sometimes, though I've gotten better. People have different sensitivities, that's all. How would she know that you were judging her? That's most likely a feeling she developed over time, but it may easily be her misinterpretation.

Anyway, don't wall yourself off simply due to an off-hand remark at a liquor store. :-)

Cheers,
Scott.