I was blessed with my oldest child teaching himself to read at age 2.5, and he is 10 and reads anything well. I would say that his vocabulary usage is above his age level, though he has great difficulty writing, both manually (he is good on the computer) and creatively, but he does have autism so I think that creativity is hard to tie in to written expression still for him. I know he will get there.
And my little one is not yet 5 and his phonics book has him already reading lots of little words. He can hardly be called "behind."
Again, I am shocked that 4 out of 5 are way below their schooled peers. What were they doing at home, I wonder?
Our eldest (age 7) sounds exactly like your eldest. My wife was researching some of his problems and we have (unofficially) diagnosed him with Asperger's Syndrome. It's nothing to be alarmed over (my wife and I both believe we are also somewhat affected as well), but more like something to be aware of when dealing with/teaching him. You might want to look into it and see if your situation would be like that.