Toushirou is in the interesting position of being almost Eren's age, but looking more like a younger kid. (Seriously, he turns 14 next month. He certainly doesn't look it because he's tiny.) Though he definitely doesn't talk like a younger kid, which probably means they wouldn't get along very well in the end because wow your views on mutants, Eren.

In any case, it seems they're in the same grade, so they should at least have a couple of classes together.