We would like your opinion if it is acceptable to use the average age of death among people who are obese and also have chronic disease conditions as causes of death. We have found that on average persons with this profile die on average at a younger age than people who only have chronic diseases. We are not sure if this is acceptable for an analysis, since we are not estimating life expectancy.
This needs further clarification: what do you mean average age of death?
You should use actual age at death to begin with in your analysis. Collect data on all deceased and stratify people based on obesity and chronic condition (present or absent). Test the hypothesis that obese people with chronic condition die sooner than the one who are not obese and do not have chronic condition.