Having a responsive site is not dependent on the version of Kentico. It is dependent on how you implement your design into Kentico. Yes there are some webparts which don't follow responsive standards but you can update most of what you need pretty easy.
For instance, take your new responsive templates markup (html, css, js) and add it to page templates, layouts and transformations. I've implemented plenty of responsive websites in older versions of Kentico. I'll say it's easier to do it in new versions because of new features but all in all, your responsive design is not dependent on Kentico's version.
There are plenty of free responsive HTML templates out there, do a google search for them and then it's a matter of implementing that design into Kentico.