wow, that's a lot of questions!!
If I have a master in something, I like him to have two skills in case I need them somewhere else. It doesn't hurt to have more than one skill.
I know a master farmer collects more food points (when he catches crabs), but I'm honestly not sure if a master scientist makes more tech points....
There is no max age for breeding.... they'll breed until they die.
Elderly just means they are running into thier golden age. I tend to put the older ones on easier jobs so they stick around longer.