//I don’t think it’s ever stated except in that Odin says in Thor 1 “My first-born” at Thor’s would-be coronation, and while I’m given to believing this is fact, it doesn’t necessarily mean it is, since, well, Odin lies, a LOT, lmao; we still don’t know if he rescued or stole baby Loki from Jotunheim, as his accounts and Laufey’s conflict, and what’s more, we never new Hela even existed because she was imprisoned presumably before Thor was even born.
Because of my studies in birth-order dynamics, which only amount to an undergrad psychology minor and therefore are not extensive–but give me at least a ROUGH idea of things– I still believe that Thor is the older sibling, but ONLY by the equivalent of a human year, maybe two. He wouldn’t be able to recall anything clearly from that young of an age.
It’s interesting, actually: Thor in some ways acts more like a youngest sibling: less goal-oriented, more socially gregarious, entitled and sheltered from criticism and responsibility. At the same time, Loki acts like a younger sibling in that he feels an anxiety or obligation to diffuse tension with humor and play peacemaker. And the way Thor hazes Loki (sometimes good-naturedly, sometimes not) everything from “know your place, brother,” to “Get Help” to “I thought you liked tricks” to the trope of no one can fuck up my baby brother but me and if you look at him cross-eyed I’ll rip off your arms, is very much big brothery.
I guess what I’m saying is, whether or not Thor is actually, chronologically, older than Loki, both of them THINK he is, and so does the whole kingdom of Asgard, and so everyone has operated accordingly in all the ways that functionally matter since their young childhood.
What confuses me more, and I used to have a theory about this but honestly can’t remember, is how Frigga feigned a pregnancy since one night she didn’t know Loki existed, and then the next, did. The kingdom obviously had no idea of his heritage or he would have learned it before he was an adult.