Ifrit acts like a child,but is most likely mid to late teens,not sure if all that [REDACTED] done by Rhine Labs had anything to do with that,she might even young adult,in human Earth years.
Eyjafjalla was mistaken for an Operators' daughter,and helped her parents before [REDACTED],which eventually led her to Rhodes Island,being a genius prodigy in Catastrophe research.
So the best guess would be that they're close in age,maybe not more then four human Earth years apart.
And of course,we have no clue how Terra does time,larger?,smaller?,then Earth,further?,closer?,to their star,are their minutes longer or shorter then ours?
This segways into how age even works,are they human-ish based or animal based?,is it species dependent?,how long can they live?
The real kicker that so many dumb people keep ignoring is the laws behind it,what's the legal age for adults?,what's the age of consent?,is there an international system or is it all city state based?
What is known,though,is that anything outside of cities and towns is lawless to Hell,so it's probably a lot more brutal,tragic,and dark then current times,maybe worse then post-Roman eras.
So it's a great thing that these two are together,for they can keep each safe enough to have a nice nap after a hard day of roasted goons.