I'm not sure actually, I am from Donostia and I did study in Batua and speak it with some minimal Gipuzkoan variations, but Batua is most like my regional dialect anyway, since it is the central one.

But I have friends from inner Gipuzkoa with a very strong accent, and it's pretty hard to understand people from Zuberoa (one of the French provinces) and particularly certain towns and small cities (not always rural) from inner Bizkaia (where Bilbo is).

Not sure how much they stick to Batua in school, but at least it seems like their family and environment has a bigger influence. Of course they understand Batua but tend to stick to their regional dialect when speaking.

But no, it is definitely not limited to the older generation.