Assumption about longevity and its consequences

If you have run into me recently, I may have bugged you with the following question:

Given the rapid pace of development in medical technology, I expect my generation to live to 100 years of age. A child being born today may live to the age of 250 years of age. Under this assumption, what health issues do I need to watch out for most to achieve that age?

I have little scientific fact to backup the assumption; it is based solely on my perception of the acceleration in medical technology today. Once you make the assumption that the average life span may be growing rapidly, you start to wonder how to take advantage of it. Or, put another way: What are the parts of your body should you be caring for most?

For example, I see three layers:

1. Mechanical stuff. If you have a bad knee, I expect that this will be fully fixable within the next 10-20 years or so. It seems to me to be a purely mechanical issue.
2. Systemic stuff. More difficult to fix, if anything goes wrong, are systemic issues, for example, arthritis or a bad lung. It is not clear to me how easily this can be fixed.
3. The brain. At the high end sits the brain. Things that can go wrong are illnesses like Alzheimer or Parkinson, but also loss of energy to live. How to avoid those?

These are all hypotheses, but the question is real. What are the most difficult things for medical technology to tackle and how to avoid that they’l become a problem once we are starting to live longer and longer lives?