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 fly 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.
If you want a more permanent cozy space, I highly recommend a bunk bed with a twin over a full. A regular bunk bed is maybe technically more fort-like, but less practical. The extra space on the larger bed below is key for not needing to hunch or crawl in; you can just swing your legs out and sit up without risking hitting your head. And it makes it much easier to have visitors as well. The main purpose of the twin lofted bed is a cozy roof, but you can have it available as extra guest space or you can convert it to storage space.
One tricky part is finding a sweet spot between a plush bottom mattress and having enough space between the two beds to hang out comfortably. I think I’d err on the side of a thinner mattress to make sure it’s roomy enough in your “fort.”