"I'd live my life much differently if I thought my brain would rot at age 50."
Not to get all Carpe Diem on you, but I'd suggest if this is true, you should probably re-evaluate your life. Whether your brain starts to rot at age 50, or you get hit by a bus in two weeks, you don't get to control how much time you have. You can only control what you do with that time.
If you live every day like it's your last then that precludes sensible things like retirement planning. On the other hand, if you know you're not going to be mentally capable of enjoying your retirement anyway, then now is the time to blow that cash.
Not to get all Carpe Diem on you, but I'd suggest if this is true, you should probably re-evaluate your life. Whether your brain starts to rot at age 50, or you get hit by a bus in two weeks, you don't get to control how much time you have. You can only control what you do with that time.