Although primarily looking at the potential predictors of language outcome, the study results published by Cristina McKean and colleagues  revealed the rather important title heading this blog entry: "Almost 19% of children (22/1204;18.9%) met criteria for low language at 7 years."
The source of the finding was a cohort of some 1900 infants "recruited at age 8 to 10 months" who were followed until aged 7 years old and subject to quite a bit of research inspection looking at "early life factors", maternal factors and "child language ability" at various points through the childhood years. I believe this was part of the The Early Language in Victoria Study (ELVS) initiative; something that has previously created a bit of stir in speech and language circles. The authors reported that alongside the quite high percentage of children who met 'low language' criteria (based on standardized receptive or expressive language scores "≥1.25 SD below the mean"): "Child language ability at 4 years more accurately predicted low language at 7 than a range of early child, family, and environmental factors." Said low language abilities at 7 years old were also "associated with a higher prevalence of co-occurring difficulties."
Aside from pointing out that language ability at 4 years old might be quite important to language ability at 7 years old, the question that should be in most people's minds is 'why?' Why are nearly 1 in 5 children presenting with low language ability at 7 years old (and presumably at 4 years old too)? Yes, there are variables such as adverse early life factors (prematurity, birth weight, coming from a non-English speaking background, etc) that will no doubt influence various aspects of language ability, but the authors note that such factors only account for roughly 15% at most of the variation in language scores seen in their cohort (not including "child language scores at ages 2 and 4"). Ergo, there are other factors involved with regards to these findings.
In light of the McKean findings, I'm also going to draw your attention back to another occasion when language ability has been discussed on this blog (see here) and specifically: "At school entry, approximately two children in every class of 30 pupils will experience language disorder severe enough to hinder academic progress."  Low language (ability) is not necessarily the same as a diagnosed language disorder, and probably accounts for the variation between the studies (1 in 5 vs. 1 in 15). But in amongst a spectrum of language ability (disorder?) the questions about 'why?' still very much remain (and please, no sweeping generalisations about us 'just being better at diagnosing').
 McKean C. et al. Language Outcomes at 7 Years: Early Predictors and Co-Occurring Difficulties. Pediatrics. 2017 Feb 8. pii: e20161684.
 Norbury CF. et al. The impact of nonverbal ability on prevalence and clinical presentation of language disorder: evidence from a population study. Journal of Child Psychology and Psychiatry. 2016. May 16.
McKean C, Reilly S, Bavin EL, Bretherton L, Cini E, Conway L, Cook F, Eadie P, Prior M, Wake M, & Mensah F (2017). Language Outcomes at 7 Years: Early Predictors and Co-Occurring Difficulties. Pediatrics PMID: 28179482