Seth's age hinges on the fact that he was a squire ten years ago when Franz's father was killed. If FE reflects the real world (and yes, that's a huge 'if'), one becams a squire between the ages of twelve and fourteen. Wiki also has people becoming knights at twenty-one but clearly in FE they get knighted much earlier. How much earlier? Well what about Franz? As far as I can tell, he's a full knight, not a squire. Ten years ago, however, he was barely old enough to remember his father. What age would you put him at there? If we go at "six" it means you can be a knight by the time you're sixteen, which is awfully young.
Interestingly, if you go with the info in the supports, Kyle has been a knight longer than Seth. Seth was a squire ten years ago when Franz's father was killed; ten years ago, Kyle was attacked during a mission to Frelia and says that his attackers realized he was a knight. That puts Kyle at at least twenty-six.
To bo honest, I just think the script writers were sloppy about details, but to write fic sometimes you have to try to make sense of the senselessness, I guess. ;P