This is a difficult question for several reasons:
From a WebMD article:
Some say the disorder is a modern problem, one that "didn't exist" when your parents and grandparents were growing up. People point to the fact that the number of American children 4-17 who've been diagnosed with ADHD increased 42% between 2003 and 2011. But does that mean more people have the condition than ever before? Again, the facts are a bit murky.
[Mandi Silverman, PsyD, a clinical psychologist at the ADHD and Disruptive Behavior Disorders Center at the Child Mind Institute replies:] "Our answer is that there's more awareness, more research, and we have more sophisticated assessment tools."
There's also significant variation even within the US; from the same aritcle:
Another ADHD-related mystery is that rates seem to vary a lot based on geography. More than 13% of children in certain states -- including Alabama, Arkansas, Kentucky, Ohio, Rhode Island, and South Carolina -- have been diagnosed with the disorder. Yet other states -- including California, Colorado, and Nevada -- report diagnosis rates of 7% or lower.
There are also big differences in the number of people who are diagnosed with ADHD around the world. For example, some surveys have found that 11% of children in Australia have been diagnosed, compared to less than 3% in the United Kingdom.
Since in the US the criteria are the same, the difference can only come from training differences of the mental health professionals applying the diagnosis and/or variation in the pool of children that are screened in the first place...
The CDC has some potential insight into the latter, with racial and insurance variation:
In 2011–2013, the prevalence of diagnosed ADHD among children aged 4–17 was 11.5% among non-Hispanic white children, 8.9% among non-Hispanic black children, and 6.3% among Hispanic children.
In 2011–2013, the prevalence of diagnosed ADHD among children aged 4–17 was highest among children with public insurance (11.7%) and lowest among uninsured children (5.7%). Among children with private insurance, the prevalence of diagnosed ADHD was 8.6%.
An intriguing explanation of the geographic varioation within the US is found in a 2013 NYT article:
Even after [Stephen] Hinshaw’s team accounted for differences like race and income, they still found that kids in North Carolina were nearly twice as likely to be given diagnoses of A.D.H.D. as those in California. [...] In trying to narrow down what those influences might be, Hinshaw evaluated differences between diagnostic tools, types of health insurance, cultural values and public perceptions of mental illness. Nothing seemed to explain the difference — until he looked at educational policies.
The No Child Left Behind Act, signed into law by President George W. Bush, was the first federal effort to link school financing to standardized-test performance. But various states had been slowly rolling out similar policies for the last three decades. North Carolina was one of the first to adopt such a program; California was one of the last. The correlations between the implementation of these laws and the rates of A.D.H.D. diagnosis matched on a regional scale as well. When Hinshaw compared the rollout of these school policies with incidences of A.D.H.D., he found that when a state passed laws punishing or rewarding schools for their standardized-test scores, A.D.H.D. diagnoses in that state would increase not long afterward. Nationwide, the rates of A.D.H.D. diagnosis increased by 22 percent in the first four years after No Child Left Behind was implemented.
The WebMD article also claims that
A recent review of studies concluded that the true rate of ADHD among children is pretty similar throughout Asia, Africa, Australia, Europe, and the Americas.
without a clear reference, so I'm guessing it might be pointing to the highly cited (>1500 in GS) 2003 review of Faraone et al.. Here's the summary of findings (keep in mind that it predated DSM-5):
Table 7 shows the range of prevalence rates for US and non-US populations for the three DSM diagnostic criteria when outlying values have been excluded. Comparison of the prevalence range for the US studies shows that the highest prevalence is reported when using DSM-IV criteria, as has previously been shown by Wolraich et al (17) and Baumgaertel et al (16). The non-US studies also showed a higher prevalence of ADHD when using DSM-IV diagnoses. As Table 7 shows, the range of prevalence reported in the non-US studies is, for each diagnostic system, similar to that reported in the US studies.
Several of the non-US DSM-III-R and DSM-IV studies found fairly low prevalence figures, between 2.4 and 7.5%. The countries included in the low-prevalence group are Sweden (2/2 studies in this population), Italy (1/1 study in this population), Australia (2/2 studies in this population), Iceland (1/1 study in this population), and Spain (1/1 study, but only for 11 years age group). This may reflect a true lower prevalence in these countries but further studies are required to confirm this. These countries are not represented in the studies using DSM-III diagnostic criteria.
Whether this exclusion is cherry picking or justifiable removal of outliers... you be the judge. The review does confirm the obvious fact that the more inclusionary the criteria is, the more children get diagnosed, regardless where they live.
A slightly more recent (2007) and even more highly cited review (>3K in GS) concludes with
geographic location plays a limited role in the reasons for the large variability of ADHD/HD prevalence estimates worldwide. Instead, this variability seems to be explained primarily by the methodological characteristics of studies
And exactly what was found to influence the results is summarized in the regression table below:
And here's the authors' explanation in words
In the final multivariate metaregression model (Table 2),
the methodological variables that remained significantly
associated with the prevalence rates were the requirement
of impairment for the diagnosis, diagnostic criteria, and
source of information. As expected, studies without a definition
of impairment had significantly higher ADHD/HD
prevalence rates than those with a definition of impairment
(p<0.001). Studies based on DSM-III-R or ICD-10 criteria,
respectively, had significantly lower ADHD/HD prevalence
rates than those using DSM-IV criteria (p=0.02 and
p=0.005, respectively). Studies that relied on information
provided by parents, teachers, and “or rule,” respectively,
were associated with significantly higher ADHD/HD prevalence
rates than those relying on a best-estimate procedure
(p=0.02, p<0.001, and p=0.003, respectively), whereas
those relying on information provided using the “and rule”
criterion were associated with significantly lower ADHD/
HD prevalence estimates (p=0.04) (Table 2).
Geographic location was associated with significant
variability between estimates from North America and
both Africa (p=0.03) and the Middle East (p=0.01). Estimates
from these areas were significantly lower than estimates
from North America. No significant differences
were found in prevalence rates between North America
and Europe (p=0.40), South America (p=0.83), Asia (p=
0.85), or Oceania (p=0.45) (Table 2).
The multivariate metaregression model using Europe
for the comparison yielded similar findings. Impairment,
diagnostic criteria, and source of information remained
significantly associated with prevalence rates. Significant
differences were found only between Europe and both
Africa (p=0.05) and the Middle East (p=0.03).
To increase power by decreasing degrees of freedom, an
additional model was run using only methodological variables initially. Methodological variables associated with
the prevalence rate for a p≤0.20 in univariate analyses were
initially included and progressively deleted from the model
using a backward procedure. Then, the geographic location
of the studies was entered. In this multivariate model,
geographic location was not significantly associated with
prevalence rates after adjustment for methodological variables
(data available from authors upon request).
Finally, since most studies were conducted in Europe
and North America (N=64), the same data analysis strategy
was applied using only these two regions. In the final
multivariate metaregression model, the same methodological
variables remained significantly associated with
prevalence rates: impairment, diagnostic criteria, and
source of information. Again, geographical location (Europe
versus North America) was not significantly associated
with ADHD/HD prevalence rates (p=0.61).
It's notable that, in contrast, if you simply look at the prevalence rates geographically without taking into account all the other variables, there does seem to be an explanatory difference between Europe and North America.
However this difference was better explained (in the authors' regression analysis) by the methodology used rather than by geography. The (sparse) data from Africa and the middle East however could not explained away by methodological variables alone. Hence the conclusion of the study that "geographic location plays a limited role".
One more thing to note is that some environmental/developmental factors affecting ADHD are known, in particular prenatal exposure to tobacco smoke. So potentially such factors could explain some/more of the geographic variation as well, although whether they do, I don't know.