Boy denied first communion due to mental disability.

1. He's not mentally "all the way there" so he probably won't remember it at all.
2. He won't be able to appreciate and take it seriously.
3. He'll have no idea what he's doing.

Well, aside from the mental part, it's not like other kids his age who do go will be exempt from these reasons either. I didn't take religion seriously when i was 8 and my aunt/uncle/their kids were heavily involved in the church. Sure as hell didn't have any idea what the first communion was for if/when i went to it. I didn't even know what the point was until i read your explanation for it just now. I would imagine most kids don't take religion seriously nowadays. Which is why it's been dying out lately.