The APOE4 allele puts you at higher risk for developing Alzheimer's disease. But earlier in life seems to boost efficiency in the brain, be associated with better problem solving and higher levels of fertility. So it's like this double edged sword. Because we're designed, evolution has set us up to be at our peaks at around 25 or whatever. And then it essentially sweeps us under the rug or keeps us around long enough so that we can be good grandparents.