I don't remember ever hearing the expression used in conversation. These days it seems to be pretty much a literary term.

I use it all the time. Meaning from about the ages 18 to 24.
I hear it, too. It is taken as meaning carefree days, before adult responsibilities became the ruling factor.
Nobody asks what I mean (and beieve me, my chums will ask) so I presume that the meaning is generally understood to be as mentioned above.