The conclusion made in the first line correlates units of alcohol drunk in middle age with risk of dementia. What it fails to consider is that in middle age, people have different occupations which could cause individual differences. (e.g. some occupations may have a higher risk of dementia than others).
B is wrong because “drinking too much is defined as more than 40 units per week” is not an assumption. This can be seen from the graph that drinking 40 units of alcohol per week has the same hazard ratio as not drinking at all.
C is wrong because the main topic of the passage is dementia. It is not making any assumptions about other health risks.
D is wrong because the passage does not say that drinking habits ONLY in the middle age increases the risk of dementia.
E is wrong because the passage does not imply whether people can develop dementia in middle age or not.