How should I adapt a recipe if using very new dried yeast versus slightly older dried yeast?
First off, how do you store your yeast?  Storing yeast in the fridge helps it to last longer; I've had some yeast in my fridge for a year that is still going strong.

As your yeast begins to become ineffective, I would personally get new yeast.  Once yeast looses its power, you're simply not going to get the same effect out of it.