Volume Orders Discounts
These result in significant discounts to the airline. While discounts are commonplace in the industry, Ryanair ordered 737-800 for an estimated 40-50 million dollars as compared to the list price of around 90 million dollars. With an order of that size Boeing could not risk losing it to another manufacturer, and will be satisfied with less profit per aircraft. Source
Some airlines sell their old aircraft after only a few years while their value is still high but the maintenance costs start to increase. Increased maintenance is bad when you try to squeeze out as many flights as possible and you have small profit margins on every flight. You can find some of Ryanair's old aircraft by the company code (AS) with other operators by searching 737-8AS.
Another suggestion is that the customers perception of Low-Cost airlines is generally negatively biased and having new aircraft counters this effectively.
Some operators (such as charter operators) will not have the same sensitivity to a slightly higher fuel consumption on their aircraft. They may not use their aircraft the same number of hours, so having an increasingly expensive jet sitting on the ground is not cost-efficient. The aircraft ordered by IndiGo are all new fuel efficient A320NEO.
Some of these ideas are emphasized in your link.
Of its 530 orders from IndiGo, Airbus has so far delivered 99 aircraft. The airline currently has a fleet of 83 planes and has phased out 16 planes that were over six year old or among the first in its fleet.
A bit of Ryanair strategy is outlined in this document.