Wow Suzanna what a difficult choice to make. It is a pity that there is no one on this forum who has gone for the test.
There is a type of dementia called Pick's disease than mainly affects people in their 50s and 60s, and is often genetic. There is an account on the Pick's Disease Support Group website written by a woman who did have the genetic test. You can read it at:
http://www.pdsg.org.uk/caregiver_stories/test_for_frontotemporal_dementia/
I have met a woman in her early 50s whose family have the Alzheimer's gene. She decided not to have the test to see if she has the gene as her attiude is "what will be will be".
When my Mum was 57, she was diagnosed with Pick's disease. A couple of years later Mum's diagnosis was changed to Alzheimer's, which is also often genetic and if it is the genetic form that my Mum has, then I have a 50% chance of having it too.
Mum is the only member of her family who has young onset Alzheimer's. Her mother had Alzheimer's in her 80s, but we were told this was just a coincindence, and not an indication that Mum has the genetic form. For various reason, the family have lost contact with other members of the family such as great aunts etc so we don't know if other relatives have or had it.
However, mum's father died in his early 50s, so there is a chance that he may have had the gene but and would have had Alzheimer's if he had lived for longer.
I decided not to have the genentic test as I don't think I could cope with the certainty of Alzheimer's hanging over me. I can cope with an "if".
I wouldn't let any of this stop me from having children. If you want children, go for it. Even if you do have the genentic form, you will have many many years of happiness with your children. And there could be a cure for Alzheimer's before they get to that age, or indeed before we do.