Today bitcoin is already known worldwide. So we dont need 6 years to be able that all people will be using bitcoin. Were living with advance technology today, and were still uprading every year. It means that even next year bitcoin will be use by almost everyone.
Well when you say almost everyone, that means majority of the people would already be at least aware of what bitcoin is. The fact that today, online access is not available or is very limited to a lot of poor countries, means that it would still take a long time for this to happen. It may very well even take longer than 6 years to make bitcoin known to majority of our population. Although bitcoin has seen awareness growing rapidly this year i think it would take more than 6years if it even happens that almost everyone will be using bitcoin. We'll never know just yet