Because the noun is definite and the adjective attributive.
en pen kone = a pretty wife (indefinite noun)
den pene kona = the pretty wife (definite noun, attributive adjective placement)
kona er pen = the wife is pretty (definite noun, predicative adjective placement)
It's more dialect dependent than it is age dependent.
Of course the older generation, having been less impacted by the media in their formative years, and perhaps also being a little less mobile, will have stronger (or purer, if you will) dialects, but the feminine gender is not the first to go. Rather, it's the very dialect specific words and phrases that do; the ones that wouldn't be understood by people not familiar with the dialect in question.