Edge’s standoffish behavior to Rydia is the source of his unrequited love for her.
On the way to the Moon, Cecil tells Rosa to stay behind. Edge follows suit, a bit more harshly:
Edge: This is work for grownups. You just wait for us back here.
Edge says it best:
“This is work for grownups.”
Ironic. He probably doesn’t know the background behind Rydia’s true age.
Rydia is only seven years old at the time.
She looks like an adult at this part of Final Fantasy IV, but she’s only seven years old throughout the entire game.
Leviathan took Rydia to the Feymarch, the home of all Eidolons, where the flow of time moves faster. While Cecil and his allies are separated from Rydia for only a short period of time, years go past in the Feymarch, and, under Leviathan’s watchful eye, Rydia grows into a young woman.
This is very similar to the concept of Time Dilution:
According to the theory of relativity, time dilation is a difference in the elapsed time measured by two observers, either due to a velocity difference relative to each other, or by being differently situated relative to a gravitational field. As a result of the nature of spacetime, a clock that is moving relative to an observer will be measured to tick slower than a clock that is at rest in the observer’s own frame of reference. A clock that is under the influence of a stronger gravitational field than an observer will also be measured to tick slower than the observer’s own clock.
This is the reason astronauts look as if they’re aging slower than the rest of us on Earth…yet they aren’t. Gravity plays a part in this. The muscles of astronauts will atrophy in space due to lack of us and gravitational pull. Here, astronauts experience a physical change due to space.
Rydia experienced a physical change due to time, but also magic.
Due to the magical power located on the Summons’ plane, time flows faster in the Feymarch.
We can see this in The After Years when Palom and Porom accompany her to Feymerch.
Rydia: The Eidolons are not found of humans going into the Feymarch. Plus, once you enter the Feymarch, time starts to flow erratically.
Palom: You mean…
Rydia: If you spend too long in the Feymarch, you’ll wind up out of sync with the time flow of the human world. Are you willing to risk that?
Palom: I am!
Palom: If we go in for just a little bit, that won’t put us too out of sync with human time, will it?
Rydia: I can’t say…
Rydia: The two of you wait here, please.
Palom: Lemme go with you!
Rydia: No! I’ll be right back.
Rydia: Promise me that you won’t follow me inside, okay?
Palom: Come on! We’re practically there already!
Rydia: You can’t, Palom. Please, you’ve got to promise me!
Porom: All right. We’ll wait here for you.
Rydia: Okay, I’ll be back shortly.
Palom: Sheesh. Why’d she even bring us here if she was gonna do this at the very end?
Porom: Palom… Why do you think Rydia brought us here in the first place?
Palom: Why? Well, because… Hmm. Good question, actually.
Porom: Maybe she wanted to test our hearts…
Rydia: Sorry to keep you.
Palom: Keep us? It was just a few seconds.
An Eidolon’s body can probably withstand the effects of time since magic seems to be outside of Earth’s spacetime. Rydia’s body isn’t exempt, and as a result, she ages faster. Upon coming to Earth, she has the body of a young adult. In The After Years, Rydia is physically 24.
But not mentally.
In the first game Rydia’s body ages, but her mental age is still the same. It’s very similar to Shelke in Dirge of Cerberus, only Shelke’s body in stays the same as she chronologically ages.
A good clue is when she reveals herself after being a stowaway on the Lunar Whale in the first game. Notice her “Tada!” pose. It’s childish, showing that she’s still seven years old. Yet, this is what Edge likes about her.
Edge: But, Rydia…There’s not a girl in this world that will ever come close to you.