Did you know that 38% of grade four students have
reading abilities below the lowest basic level as determined
by the National Assessment of Educational Progress (NAEP)?
The NAEP is the only ongoing survey of what students knew 
and tracks their performance in various academic subjects
 for the United States. 
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In their report, the NAEP found that 38% of grade four students had reading achievement below basic levels, with a basic level reading score being 208.
To put things in perspective, the US reading scale has an upper limit score of 500, with average reading scores for grade 4 (217), grade 8 (264), and grade 12 (291). The grade 4 reading achievement levels are categorized by the NAEP as Advanced (268 scores), Proficient (238 scores), and Basic (208 scores), and the basic reading achievement level is defined as follows by the NAEP:
Fourth-grade students performing at the Basic level should demonstrate an understanding of the overall meaning of what they read. When reading text appropriate for fourth graders, they should be able to make relatively obvious connections between the text and their own experiences and extend the ideas in the text by making simple inferences. [1]
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Unfortunately, over a third of all grade, four students read at levels even below basic. Is your child having reading difficulties? Research on Phonemic Awareness has found that early reading helps improves a child's reading and spelling abilities. 
In fact, the National Reading Panel has concluded based on their massive review of over 1,900 studies that teaching phonics and phonemic awareness produces better reading results than whole language programs.
There are numerous documented benefits and advantages of teaching children to read early on, and teaching them to read using phonics and phonemic awareness instructions. 
It is clear that early language and reading ability development passes great benefits to the child as they progress through school at all grades, and that early language and reading problems can lead to learning problems later on in school. 
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For example, a Swedish study found that children with a history of reading problems at school entry scores significantly below average on reading in grade 4. As well, children that show very low interest in books and story reading before age 5 also scored similarly low on sentence reading in grade 4. [2] This is just one of many studies which have similar findings, and this makes it an imperative for parents to begin exposing their children to books and reading at an early age.
Good question!
There's no set guideline on when you should start teaching your children to read; however, you can start cultivating your child's love for books and reading as soon as they're born. Obviously, very young babies would not even know what books are, however, talking to your child and reading to your child will help them develop a keen liking for books and stories. 
How to help children develop a love for reading? | Learning 4 Kids
As your child grows and gets older, avoid TV-sitting them because as they develop a dependency on television as their main source of entertainment, it becomes very difficult to dislodge that need for TV entertainment and get them to enjoy reading books. Instead, keep age-appropriate books all around the house, and read to them often. You'll find that they'll start picking up books and pretend to read themselves, although, at very early ages, they still cannot read.
People typically think that kindergarten or grade one would be an appropriate time for their children to start reading; however, this is not the best approach as studies have repeatedly found that children with good phonemic awareness before entering kindergarten continues to outperform, and achieve exceptional reading and spelling abilities as they progress through school. 
On the other hand, children who enter school with reading difficulties may continue to have reading and spelling difficulties.