IPv6 has been available for a long time, but adoption is far from 100%. Google sees a huge percentage of the world's web traffic, and collects statistics about the way that we use the web - including how many people are using IPv6. The results are interesting, and show how poor IPv6 adoption is in some parts of the world.
According to Google's statistics, fewer than 14 percent of the web as a whole have good IPv6 connectivity. There are some territories that have 100% adoption, but even the United States only has around one fifths of its users on IPv6 compatible connections - and in the United Kingdom that figure is closer to ten percent.
This is going to be a serious problem in the future when more people start using IPv6 to power their websites. The web is running out of IP addresses for devices as more and more things connect to the Internet. It used to be that each home may have just one PC, but now there are games consoles, smartphones, laptops and tablet PCs connecting to the Internet as well - and web TVs and other streaming devices, and even 'smart appliances'. This means that there just aren't enough addresses to go around for the number of people in the world.
IPv6 was designed to stop that problem - but there is a lot of hardware out there that just isn't IPv6 compatible,and people who are using that hardware will struggle to connect to IPv6 enabled websites. There are workarounds, but they're not ideal - and nor are they sustainable.
One of the biggest issues is that IPv6 is not backwards compatible with IPv4 - so this means that we are going to need to find a way to encourage better adoption. Right now we have a situation where some service providers are reluctant to implement IPv6 because content just isn't there, and content providers don't want to use IPv6 because there aren't many service providers using it. Yes, it's possible to offer connectivity on both, but justifying that is hard for time-strapped companies that are more interested in getting their content seen than messing with technology.
There is still an upward trend for IPv6 adoption, but it is perilously slow, which is unfortunate given that IPv4 addresses are a finite resource, and we do need to solve this problem for good.