What it says on the tin.
Alright. General Seth of Renais, aka the "Silver Knight," appears at first glance to fit neatly into an archetype box. Older sources will assure you that Seth is "an Oifaye," aka "a Jagen that stays good," but these are now classified as fictions based on a misunderstanding of Jagen's usefulness in FE1 and an inflated sense of what Oifaye can do in FE4. Stats aside, though, he's definitely appears of a sort with Oifaye, not to mention FE3's Cain, FE5's Finn, and Seth's successors Titania and Frederick in FE9/10 and FE13. All of 'em are veteran fighters who fall somewhere in their mid 20s to mid 30s-- definitely with an edge of age and experience on the teenaged main cast, but not, you know, Jagen-Mycen-Marcus levels of old
Seth is definitely top dog among the knights of Renais-- here
he is with the resident red/greens, Forde and Kyle, plus Forde's little bro Franz, the requisite n00blet knight. One would assume Seth to be the eldest of the four by some years.
Except there seemed to be some textual evidence suggesting that Seth might actually be younger than Kyle (and presumably Forde) and must be one hell of a prodigy. This hypothesis was advanced by korsriddare back on the old magvel comm, and I'll let his evidence speak for itself.
Now, FE8 never gave us concrete ages for anyone but Myrrh and Dozla, so this could certainly be true. Seth arguably looks older than Kyle'n'Forde, but art can be misleading. Also, Seth-- unlike all the rest of the Jagens/Oifayes/whatever you wanna call 'em, can get with his leading lady, which might imply he's actually rather close in age to Eirika. Wouldn't hang my hat on that one, though-- FE doesn't shy away from May/September kinda relationships. (Oifaye has a game-endorsed pairing with a lil' pegasus knight who's probably twenty years younger than he is, and let's not even get into what Frederick can do.)
So really, it's down to sifting through these little clues from the support conversations. Does the death of Forde'n'Franz's father ten years ago really imply Seth was a mere apprentice at that time? And was Kyle truly a fully-fledged knight, possibly Seth's senior, at that same ten-year timeframe?
Well, I ran the Kyle/Syrene supports by the friendly local FE Japan translation tumblr, and it looks like NoA look a few minor liberties with the text. Nothing earth-shattering to the meaning of the conversation, but the original text just doesn't state that Kyle was a knight ten years before the start of the game. He was a horseman, yes. Running errands, yes. Being very brave, yes. A knight, not necessarily (and Syrene apparently wasn't either but that's not clear). Kyle could just as likely have been an overly-conscientious n00blet himself at the time. He was delivering a letter, fercryingout loud. Even in "peaceful" Magvel I'd hope they weren't wasting the time of a fully qualified knight with the royal mail service-- there's brigands and pirates and whoever the hell keeps hiring Jehannan mercenaries to worry about. But an apprentice of, say, thirteen or fourteen who knew which end of the lance to hold and wasn't inclined to get into trouble might well have been entrusted to deliver a letter by himself. Same goes for Syrene, who could've been a Florina/Shanna/Karin type at the time, able to fly and fight but not properly a knight.
[It was Kyle's statement about "I had no idea you'd become a knight!" to Syrene that got me on this track again. This Frelian chick bailed him out of trouble... and wasn't a knight. Just some random teenager with weapons and a flying pony? It made me wonder if Kyle himself weren't exactly a proper knight in shining armor at the time of this bandit encounter... and, per NoJ, he may not have been.]
In conclusion... d00d, this is Magvel. At the end of the day we still don't know anything! We don't know whether or not Seth was still a "mere apprentice" when Forde'n'Franz's father died. We don't know whether or not Kyle was a qualified knight during that same time period. We don't know how old they are in relationship to one another or to anyone else beyond that yes, they're both clearly younger than Dozla and Myrrh.
Unless Seth's not fully human and is actually, like, sixty or something. At this point I'd believe almost anything.