You have as much chance of getting AZ. as you do getting hit by a bus while crossing the street. They actually don't know who gets it and why or when. You will better your chances of not getting it by staying mentally sharp and keeping your brain alert. Play mind games, read, stimulate yourself, stay fit both physically and mentally. Worrying won't do you a bit of good.
There are making great strides in the treatment of this horrid disease so if by some chance you do get it, they may have a cure, or have found a drug that will keep it at bay or at the early stages for decades. I lost my mother to the disease and my dad had vascular dementia, but I refuse to worry about that. Life is too short to spend my time worrying.
If you want more information, contact your local Alzheimers Association. They have a wealth of information that you can have. It will fill you in on all the stages and the research going on right now.
I echo everything IBake said. My grandmother has Alz and my mother has it now. Several of grandmother's siblings and Mom's half siblings also had it. But there are many that don't as well. My dad has vascular dementia. But there is no assurance that I will or will not get it. So I will just keep myself physically and mentally fit, have all my legal affairs in order, and enjoy life without worry. There is no reason to worry about what you don't know.
My mother died of it, but I am convinced that the new research is about to find a cure. By the time I get it - if I do - there will be a drug to reverse the damage.
Enjoy your life and don't let 'possible disasters' spoil your days. You know what they say - the past is gone and cannot be changed, we don't know what the future will bring and can't really do anything about it --- but we have NOW to be the best we can, help others, make the most use of this gift, which we call The Present.