Gun to your head: can you tell me which one is Rod and which one is Todd?
I drew Todd here a couple of weeks ago, and as I sat down to write this I thought to myself "Todd...he's the older one...right?"
*BUZZER* WRONG
Todd is the younger one, Rod is the older one, although even this is a contentious issue, as even the writers of the show appear to have difficulty telling them apart. In the episode "Homer Badman" the statement "the older Flanders boy is Todd, not Rod" appears in a list of corrections, although this appears to be a deliberate attempt to rile up the die hard fans who would even bother to pause for this.

While there are minor character differences between the two brothers, they are largely used as an extension of Ned's personality. They are the perfect well behaved kids, not meant as much beyond being a contrast to Bart and Lisa, and and occasionally a statement on the impressionable nature of kids. This can go one of two ways: "it's ridiculous that anyone can think that external negative influences affect a child's behaviour" and "a child's behaviour is obviously affected by external negative influences." You'd better believe the show takes both stances, depending on the joke.
Example of the former:

Example of the latter:

I'm not condemning the show for this, but it is amusing to me. This show is full of contradictions. Like which brother is older.
This post is part of my "Every Simpsons Character Ever" series. For a list of my rules in this project, click here.