ps! Most successful LCC's use new airframes because they treat them like a consumable.
Ryanair buys airframes at such volumes that it gets them for very good prices, so much so that they flog the crap out of them, do as little maintenance as possible over the course of their life (I.e. If you were going to keep an airframe 15-20 years you would do a lot more preventative maintenance) and roll them over at 8-10 years before the big checks come due. When they do flog them they sell them for as much or more than they bought them for I.e. They break even or make a profit on a cash basis for each airframe.
Not sure about Easyjet. southwest are different as far as I am aware, they buy a mix of new and used airframes so their fleet profile is different.