Not to pile on you here, but these students are also 2 years older and hence 2 years more experienced casting magic. Considering a full education for most is their 7 years at Hogwarts, those extra 2 years can make quite a difference

 

I don't think it's unrealistic to say that Harry with Hermione doing research for him could master any relatively low-level spell with some dedicated effort and time; Harry learns many of his most useful spells outside of class under circumstances that have nothing to do with how good of a wizard he is or how old he is, and whenever he dedicates himself to learning a spell he's generally capable of getting it down. Stunning seems totally out of reach for an average first year, but the Disarming Charm seems plausible enough if Harry has time to prepare and is completely convinced he needs it to beat Malfoy or whatever.