My younger daughter is dyslexic, and we noticed differences in her language development long before the typical ages for reading. Things like difficulty with rhyming and the inability to come up with the word she wanted. For example, she often couldn't recall the name of an object and would give a long description until you guessed it correctly. She knew the the word she wanted; she just couldn't pull it out of her brain. If you notice other language difficulties -- I think there's a list on Susan Barton's website -- you might want to look into testing. But late reading alone doesn't necessarily mean dyslexia.
We've been using Barton for a few months and I think it's a great program. Very thorough -- actually too thorough for our needs. We're now going to switch to All About Spelling (another Orton-Gillingham-based program), since DD's issues mostly affect writing rather than reading. But a friend's daughter, who has more severe dyslexia, has shown great progress with it.
HTH,
Margie