Purpose Lines are from the edge of the bottom of the nostrils and go down to the outer edge of the mouth, sometimes called “marionette lines”. Purpose lines have to do with being on your true path. The lines should definitely start to be there after 40 but shouldn't be too deep or too shallow. The left side has to do with someone's inner true path and the right side is what they do publicly, maybe their job for example. Sometimes people, situations, even ourselves block us, those are the "boulders" in life to overcome to get to where our soul is calling.
In the image you can see this woman's purpose lines are showing up pretty well even though it's a side view gives an idea of where they are located and what they might look like. Unfortunately, society doesn't value these lines and often people have them minimized by fillers which can diminish finding and getting on the golden path they are meant for. Sadly, if they are not consciously aware of the consequences of filling in purpose lines can throw them off.