I don't think people grow out of their love of forts, it's just that they physically grow out of their forts. Solution: put your tent's footprint underneath an area rug and erect your tent just using the rain fly for a quick and easy cozy space. Great for kids and adults alike.
My Half Dome 2 Plus easily fits a full mattress/mattress topper with room to spare, so it can be used as a napping space or a place for a guest to stay and have a bit more privacy. You can drape a densely woven blanket over the whole thing to create more of a dark cave and you can add a clip lamp or something to give it more ambience.