This is a garbage article, wtf am I reading. Is it saying that irrespective of your current age, your birth year can predict an 8% variation in the mass of you

This is a garbage article, wtf am I reading. Is it saying that irrespective of your current age, your birth year can predict an 8% variation in the mass of your brain (with no effect on skull size)? Or is it saying that older people have less mass in their brains (loss of grey matter with aging has been known about for decades at this point right)? If the first one, that's way too much to explained by evolution within generations and there'd be no reason to presume it's benign at this point. It'd likely be either explained by some environmental issue either now or with past brains.