In developing countries, the potty training of the vast majority of children is accomplished by the age of 18 months to 2 years. This was true also for Western countries, 50 or more years ago:
Studies released by Harvard University in 1956 revealed that the potty training of 80% of American kids was accomplished by the age of 2 years.
Nowadays in the Western world, the age at which the potty training of just 50% of toddlers is accomplished is close to 3, and 26% of children are still not trained by the age of 4!
This is due to Diaper Dependence. You see, the only really significant change to have occurred in the toddler world over the last 50 years or so in developed countries, is the shift from wearing uncomfortable cloth diapers to the modern disposable diaper. The technology in disposable diapers has increased to the extent that young toddlers today have No Concept of the feeling of being wet, so they don't have an incentive to get out of them! So the earlier the better.