That depends on how you define it!
According to current definitions, half of American adults over 65 are diabetic or prediabetic.
More interestingly, according to extrapolations from the SEARCH study, roughly 1 in 400 people under the age of 20 in the United Stateshas diabetes (including all types of diabetes), for an estimated 215,000 youths. The estimate they were putting out last year, based on 2007 data of how old people in the US are plus data on diabetes prevalence from about ten years ago, was 186,300 youths, which was a little more than 1 in 500.
Frankly, the data is not exact enough to get a good guess. But it is likely that between 1 in 300 and 1 in 500 Americans is or was diagnosed with type 1 diabetes by the age of 20.
In other countries, the risk is fairly different. Prevalence in Finland is greater than 1 in 200, while prevalence in some parts of Asia is less than 1 in 1000.