It doesn't always work but at least you have a chance - especially on hard. If I am going to be away for any long length of time I check all my elders as doctors - they will still do research, build or farm if that is what they are more skilled in but they take more rests so don't seem to weaken as quickly.
Since trying this strategy I don't have many who weaken -if they lose health bar they are still healthy and it just means keeping them at the bin for a while.
If I have any kids that will turn 14 while I'm away then I check healing and when I check in next they are often trainee doctors and so I can set them to something else.
Hope this helps.