Near the beginning of the movi,e when Robbie gets stood up at his wedding, his friend says that he wanted to get married since he was in the third grade. His sister steps in and says that that was when their parents died. Later, in the movie, though, Robbie says that he was 10 when it happened. One should be 8 or 9 when in the third grade.
He could have very possibly been 10 in the third grade. I was only 8, but a know lots of people that were 10 in the third grade. Good catch, but it's very possible.
Different states have different education levels. Up in Washington state, they have a mandatory grade level that is like a prekindergarten. My cousin lived up in Washington until he was in third grade (at 10), while I was in fourth grade. We are only three weeks apart in age. So it is possible to be in third grade at 10, any way you look at it.