I have "almost-7" (as they make sure that I refer to them!) twin boys, and an 8 year old daughter.
They do still play make-believe, but the format has shifted dramatically. It's growing less and less about objects (lego guys, barbies, animal figures) and more about writing (making books and comics, and then plays based on those stories). THey also talk a lot about different lands and scenarios they have created, just verbally, sometimes without acting out or any props at all.
I know a lot of older kids and teens that do the same. Just because they drop the external props, IMO, doesn't mean that the teen who loses herself in her artwork, poetry, or short stories/novels isn't playing make believe. I've certainly never given it up, and I'll be 36 this year! When I am working on a storie, I get great pleasure from thinking about it, imagining scenes, making up and playing with characters in my head just as if they were dolls, ect. I just don't have the external props.

Maybe we should have a poll to see how many of us mamas like to play for playing's sake, that doesn't really have too much to do with our kids. (though I admit, I let my kids join in MY play sometimes. Just like they sometimes let me me into THEIR play!)