Basically, its like shin-chan, nobody ever leave school.
Well,some later chapters do have some "hypothetical" stories of Shin & friends attending elementary school,with a bit grown up Himawari too. Some times certainly do pass in that universe,most notably the bomb-out house and it's rebuilding effort. Too bad we won't be seeing how it'd go from there...
Includes new material. By the way, the explosion scene on the first page has nothing at all to do with the new series. In Mamange Eternal, Hime-chan is going back to being a pre-schooler, so please watch for it tepidly.