My pentas are about 2 years old 3 feet tall. I dead head regularly and blooming continues. However, the plants are getting woody and not so lush,, Should they be cut back?
Yes, if they are getting woody you can cut them back to within a few inches of the ground in the fall. Since spring is coming up, it won't hurt to cut them back now, as far as you need to.
Here is more:
https://www.gardeningknowhow.com/ornamental/flowers/pentas/pruning-pentas-plants.htm