Snipped from [Full Interview]
Fusionx: What is the difference between a Primal and an  Elder Primal?
Koji Fox: Basically it’s all a matter of 
length of  worship.  With primals, it’s all about how you have someone worshiping  you and  you become that primal and they believe in you and you get your  power  from that. For the Ixal or the Amalj’aa, it’s the belief that  makes the  aether combine to summon forth these primals—so when you look  at  Ifrit, and Titan, and the one’s we’re calling primals, they’re   considered very young. They’ve been believing in them for a long time,   but looked at in a span of 
all the eras it’s not that much  time.<...>
Anwyll: Originally, we thought it might be more  complicated than that, <...> but it’s just has to do with the 
length of time?
Koji Fox: Basic length of time.