Here are some skill and status related questions from me...

If a villager has mastered in one particular skill, should I keep him/her at it or should take him/her to master in someting else instead? And if I leave a Master Scientist doing research, will that accumulate more points than if I switch to just an adept or trainee scientist?

Is there a maximum age for breeding, and is it the same for men and women?

What does "elderly" status mean/do? If a villager is in that status, is there any way to help him?

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.

"by easier jobs", do you mean researching?

Currently I have 6 adults and 2 kids in the village, the adults are in their early 40's, and the older kid is about 11. So I have been wondering if I should add more kids into the village at this point.

Master scientists will produce more research points than less skilled scientists, so if you need lots of research points for the higher tech levels, it's better to keep your master scientists at the research table.

Humm... plenty of food... what counts as "plenty of food" for a village of 8?

And how about Healing? Is that something worth spending time on training or should I just wait 'til someone's sick and gain it that way??

Any time the food bin is approx. over 400+ food, the villagers don't stress so much about it. I always try to have much more than that if possible.

I like to have at least one villager have some healing ability. They will learn as they heal, but they will take less time doing it if they are really adept. My masters seem to make them "more" well then the non-masters.

It seems that my villagers are watering/planting/harvesting from one area only, when will they start reaching to other parts and plant crops in more than one area? Or will they ever do that?

Only one field? What about that one field where they keep "trying to water"? The one with dead flowers.