I think that phrase, do I feel like a grown up in this outfit, is a starting point for the dress (or skirt)--if you start by looking at really nice materials, figure-flattering cut, and adequate skirt length--doesn't have to be midi, but probably not really much above the knee--and then of course, complementary colors (and your boots are versatile for many color combo's) --then you can probably put together a very nice outfit. Think about what types of dress and skirt/top formulas, silhouettes, or proportions look good on you to begin with and branch out from there. I think it does help to pair not too body-con outfits with such boots, and fortunately the style trends now have loads of "fluid" items and looser, easy fits.
If leg lengthening is a focus, you may want to wear some kind of tonal hose or tights with the boots.
I tend to like how flowy or flippy skirts or dresses look with cowboy boots--the romantic cowboy juxtaposition--, but tube-type skirts with a boxy top could work, too. For fall and winter, lovely soft drapey wools in rich colors would look great with boots. Or a chiffon-y skirt with a soft sweater. Again, think about what looks good on you and just work the boots in.
I'm not on pinterest, but I expect there are a lot of photos out there and some hopefully are in the more "elegant" or arty-chic mode and not all junior fashions.