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I'm from the UK and I hear "flabbergasted" reasonably often, more often from upper-middle class folks. Maybe a regional thing?

more of an age thing? I use it but I doubt my daughter ever does

I was born and raised in the midwest part of the U.S. and I've been using "flabbergasted" for all my adult life (20+ years). Of course, I've always had a propensity for peculiar or anachronous verbiage.

flabbergasted is very widely used in the UK, really puzzled how you could possibly not come across it if you've spent more than a few months here.

Indeed. One might say it is almost flabbergasting!