I've thought a lot about this issue regarding adults and children with SN and communion.
I was reading in Christianity magazine about a boy with Downs Syndrome who had been refused admission to communion in his church (may have been confirmation?) whereas all his peers were admitted (think it was RC church)
See this I cannot get my head around at all, because I see communion (and baptism) as signs God has given us to partake in, not necessarily only for those with a great depth of understanding (or even a shallow one). None of us can say we have a perfect theological understanding - to me faith is enough, and actually, as a parent, faith of a parent (when it comes to infant baptism.)
As far as admission to communion at 7, mine were both fully up for that in their own understanding - I remember DS saying 'but why can't I have the bread, I want to thank Jesus for dying for me too?' Indeed why?
DD was confirmed last year at age 10, after being admitted to communion at 7. I was fully in favour of her doing so and it was fully her choice. Her faith is solid, she wanted to publicly proclaim her belief and re-iterate the baptismal vows made on her behalf. I personally don't think there should be an age limit, she had the understanding of a 10 year old, and this is no 'better' than the understanding of an 18 year old in faith terms. Similarly, with people with SN/brain damage etc, if they feel they want to make such a step, their understanding is a good as anyones, their faith as strong - and we can exercise no judgement over what they can/cannot do. in my opinion 
I completely agree Holo with your contention about those holding a grudge etc - I'd also say those wilfully carrying on in a certain sin while knowing it is wrong in God's sight. But as far as those who are giving communion go, they are not in a position to know such things, and so in good grace and spirit offer communion to anyone who loves the Lord. It's up to the individual to be in that place to take it.