How to explain disability to kids?

How do you talk about a disability? When is the right time to start explaining to your child that he has a disability? When is the right time to start explaining to your other child that his brother has a disability? What do kids understand when you try to explain away a disability? And how can you make sure they don't end up defining themselves through that explanation? I think these questions haunt every parent who has a Special child. Some day my child will come to me and ask me - why? Or worse, he will hear something from someone else and then will come to me and blame me for not telling him sooner, for not making sure he is ready.
I am not different here, why would I be? It is one of my biggest fears having Yon hear something about his Ocular Albinism from someone else. Having him subjected to the "albino" or "blind" or worse name calling, having him think he is less. And on the other hand the possibility that it will happen is so slim. He doesn't look albino, or blind. Just sometimes weird and funny. So why would I rush it? Am I not running the risk of doing more harm than good if I rush to explain to him things he isn't ready for and he doesn't understand?

For his part, Yon never asked us why he needs glasses. He has his glasses since he was about 10 months old, and always loved them. They are a part of him as any other body part, and never stopped him doing anything he wanted, so he never really questioned their presence. Lately they have even become his "special feature" - Ron has freckles (or spots, in Yon speak), I have glasses - which have made them even more desirable. We tried to ask him why he needs his glasses, and his answer was short and to the point - because I love them.
He seems happy enough with that explanation, and in fact refused to accept any other explanation for a long while. So are we wrong to leave it like this? We did try to explain to him that he needs them because he sees less well than Ron, and so the sun won't hurt his eyes. As a reply he told us that the sun warms his toes and he needs to take his socks off. Classic Yon for "I am not going to discuss this with you so all you will get are random sentences that will annoy the hell out of you".
We took him to meet other children with Albinism at our Outreach program, and he really was very happy there, but other than "he has glasses like me!" he didn't see any resemblance between him and the other kids, nor did he care why he is going there, or who they are. He just enjoyed playing games and running around.

But then there is Ron, and here is where the story becomes much more complicated. Because Ron needs the answers. He hears us talking, he goes with us to the Outreach program gatherings, he comes with us to hospital, he has the need to know everything anyway.
And so we explain some of it every time, but it is so hard for him to really understand, and we try to keep it on the "not really serious" side of things because he also has a tendency to worry too much and whenever Yon hears his name in the conversation he has to start talking, mostly about unrelated things, so it makes having a serious conversation a tad harder.
Then we sat down to eat and Ron heard us talking about wanting to take Yon to watch a football match and needing to find tickets in the first row, and our letter to the disability team at Arsenal to get that, and he started asking us questions about Yon's eyes, and what disability is, and he was really intent about it and about the fact that Yon won't get any better. He is studying about the body and so he wanted to understand the mechanics behind Yon's condition and its name and he found it really baffling. I understand why it is weird to comprehend the fact that Yon really doesn't see well because he isn't accident prone and he doesn't let you feel that he has a problem or difficulty. And why it is tough to understand how his glasses don't really help him, or what 40% vision means.
So we told him he really doesn't need to worry about all this, and that it is fine to not really understand everything, and he looked at us and said "I want to understand more about Yon's eyes because I want to be able to help him".
And melted our hearts.

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