Brain age was also the single best predictor of overall mortality in that analysis, published in Nature Medicine in July 2025. APOE2 carriers, whose genotype is protective against Alzheimer's, showed the inverse pattern. Among people carrying two copies of APOE4, the highest-risk genotype, extreme astrocyte aging roughly tripled the risk of developing Alzheimer's. Youthful astrocytes tracking with lower Alzheimer's risk does not establish that the astrocytes are doing the protecting, nor does it identify anything a person can do to keep them young. Where Brain Age Research Actually StandsSeveral groups are now estimating brain age from different signals, and they do not agree on a standard.