Approximately 15 - 20% of the population have dyslexia to some degree (mild, moderate or severe). That means on average 2-4 kids in a class of 20.
Dyslexia is the #1 cause of reading failure and school dropouts in the United States.
"Late bloomers" are rare. Waiting to see if a child outgrows his reading difficulties rarely works. Why? Because skill deficits are almost always the cause, and are overcome through appropriate intervention. The "wait to fail" philosophy doesn't work.
80% of students classified as LD have dyslexia. However only 1/10 of those with dyslexia are severe enough to qualify for special education.
Very few children exit special education.
90% of dyslexic children will read normally if they receive the right kind of assistance by first grade.
Studies show that almost 90% of poor readers in first grade will remain a poor reader by the end of fourth grade and beyond, without appropriate early reading intervention.
If help is delayed until fourth grade (rather than in late kindergarten), it takes four times as long to improve the same skills by the same amount. (Susan Hall, coauthor of Straight Talk About Reading)
Phonemic awareness is the core and CAUSAL factor that separates normal readers from disabled readers. (G. Reid Lyon)
Phonemic awareness skill deficiency can be accurately tested for in children as young as age 5.
Conclusion
We know what it is, we know how to identify it, we know how to treat it, and we know the ramifications if we don't. So why aren't we doing it?
Sources
National Institutes of Health (NIH)
http://www.commercialappeal.com/news/2009/oct/22/guest-column-early-diagnosis-intervention-key-in/
http://www.dys-add.com/LateBloomers.pdf